The identity in development and structure of the placoid dermal elements (shagreen denticles) with the teeth, in Elasmobranchs, was recognized in Williamson, W. C. 1849.1, and elaborated in Hertwig, O. 1874.1.

During the inpushing of the outer skin in the formation of the stomodæum, these placoid elements (teeth) have been carried into the oral cavity. While teeth are generally restricted to supporting bones or cartilages, in numerous species of sharks and rays, practically unmodified placoid scales persist in the lining of the oral cavity and pharynx.

For references, see Imms, A. D. 1905.1; Spengel, J. W. 1905.1; ★Steinhard, O. 1903.1.

Similar oral and pharyngeal denticles in Chimaroids are figured in Dean, B. 1906.1, p. 121.

The formation of a triconodont tooth (in Chlamydoselachus), through the fusion of

three separate simple teeth, is shown in ★Röse, C. 1894.2.

Histology of teeth

Histological structure and development of teeth. Born, G. 1827.1; Fischer von Waldheim, G. 1801.1; Heincke, F. 1873.1; Hilgendorf, F. M. 1888.1; Morgenstern, M. 1909.1; Owen, R. 1839.4, 1840.3;

Röse, C. 1897.1; Schwalbe, G. A. 1894.1; Waldeyer, H. W. 1864.1; Huxley, T. H. Add. 1853.1; Owen, R. Add. 1839.1.

ENAMEL

As early as 1848, M'Coy (Sir F. 1848.1, p. 124) following the then recognized fact that the enamel of fishes differs in development from that of mammals, applied the term ganoine to the layer of "false enamel on the teeth of certain fossil sharks (Chomatodus). As pointed out to him by Prof. Owen (Owen, R. 1849.1) the latter's term "vitrodentine" had previously been in use. About the same time, Williamson (W. C. 1849.1, p. 438) independently, but more aptly, used the term "ganoin" for the enamellike substance of the scales of Lepidosteus.

While the exact nature of enamel seems not yet entirely settled, a recent authority (Tomes, C. S. 1900.1, p. 62) retains the term "enamel " which he divides into the following types.

(1) Enamels not wholly epiblastic in origin. The stroma which is the seat of enamel calcification is furnished by a transformation of the exterior of the mesoblastic dentine papilla, the ameloblasts apparently segregating the lime salts required for its hardening. (This type is found in Elasmobranchs and is unquestionably a modified dentine vitrodentine of

Owen.)

(2) Enamels wholly epiblastic in origin, in which the ameloblasts undergo a prior transformation into a stroma of the dimensions of the finished enamel and themselves disappear. This type is met with in the Gadidæ, in Sargus and in Labrus and probably other fishes.

(3 and 4) The other types, also epiblastic in origin but in which the ameloblasts persist, are confined to the mammals.

For papers specifically relating to development and structure of enamel, see Nunn, E. 1883.1, and Tomes, C. S. 1898.1-1900.1.

DENTINE

Although accurately described as early as 1836 by Retzius (A. J. 1836.1), the various forms of dentine were not named until the ap

pearance of the "Odontography' of Owen (Owen, R. 1840.1) whose terms are now generally accepted.

As more precisely defined by Tomes (C. S. 1878.2), the forms now recognized are (1) hard unvascular dentine, (2) vaso-dentine, (3) plicidentine (Labyrinthodon, not in fishes), and (4) osteo-dentine.

Structure and development of dentine. Korff, K. 1910.1; Mummery, J. H. 1893.1; Studnička, F. K. 1906.2, 1909.2; Sternfeld, B. Add. 1882.1; ★Tomes, C. S. 1878.2.

Cornified epithelial teeth of
Cyclostomata

In this group the teeth are purely cuticular, formed by an axial dermal papilla, invested by epidermis and an external horny cone. They are without genetical relations to the teeth in other groups.

In Murine and Bdellostoma, according to Beard, the dermal papilla develops an imperfectly calcified tooth beneath the epidermal cornification.

Dentition - Cont'd.

Development, structure, and arrangement of teeth in various genera, chiefly Petromyzon. Beard, J. 1888.7, 1889.1; Behrens, G. 1891.1; Jacoby, M. 1894.1; Renaut, J. 1900.1; Scott, W. B. 1883.2; Tims, H. W. 1906.1; ★Warren, E. 1902.1; Studnička, F. K. Add. 1900.1.

Dentition of Plagiostomi

Numerous genera of fossil sharks, whose relationships, in many cases, are but imperfectly known, have been described from isolated teeth. It is obviously beyond the sphere of the present work to adequately refer to these genera. They are, moreover, carefully treated in Woodward's "Catalogue of the fossil fishes in the British Museum (Woodward, A. S. 1889.2), to which reference may be made.

There are a large number of Pre-Linnæan references to isolated fossil shark teeth, which were known as glossopetræ, and to the fossil teeth of pycnodont fishes known as bufonites (toad-stones, i. e., stones supposed to have been formed in the head of a toad). These are brought together under Palæontology.

The myth of the toad-stones is discussed in Hussakof, L. 1915.3.

Important treatises in German, chiefly inaugural dissertations, on the development and histological structure of Selachian teeth. Benda, C. 1881.1; Hertwig, W. A. 1874.1; Jentsch, B. 1897.1; ★Laaser, P. 1900.1, 1903.1; Owen, R. 1839.3, 1840.4; Röse, C. 1897.1; Treuenfels, P. 1896.1.

Numerous shark teeth were dredged in the tropical Pacific by the Albatross. See Eastman, C. R. 1903.6, 1906.3.

Histology and development of the rostral teeth of the sawfish, Pristis. Engel, H. 1909.1; Green, J. 1859.2; Hilgendorf, F. M. 1888.1; Kölliker, R. 1860.5.

Peculiar modified flattened posterior teeth occur in the Port Jackson shark, Cestracion. Miklukho-Maklai, N. N. 1879.1.

Dentition of the eagle rays, Myliobatida. These possess both an upper and lower median series of modified, flat, pavementlike, crushing teeth. Gudger, E. W. 1910.2, 1914.1; Harless, E. 1847.1; Stefano, G. 1914.2; Stromer, E. 1904.1; Treuenfels, P. 1896.1.

Sexual differences chiefly in the shape and size of the teeth have been described in the following forms. Raja. Lütken, C. F. 1874.3,.5. Notidanus (Heptanchus). Macdonald, J. D. & Barron, Č. 1868.1. Mobula. Pellegrin, J. 1912.12.

The evolution of sharks' teeth. Woodward, A. S. 1892.4.

Fine figures of the teeth and of the jaws with teeth of Plagiostomes may be found in Garman, S. 1913.1.

Descriptions of individual teeth or the complete dentition of the following forms. - Carcharias. Ayres, W. O. 1849.5. Rhincodon. Bean, B. A. 1905.2; Gudger, E. W. 1915.1. -Hexanchus. Carruccio, A. 1896.1. Notidanus primigenius. Probst, J. 1858.1. - Chlamydoselachus. Röse, C. 1894.2. Mustelus. Spengel, J. W. 1905.1. Lamna. Stannius, F. H. 1842.1. Cetorhinus. Turner, W. 1879.2.

Miscellaneous and usually unimportant papers on sharks' teeth or the dentition, succession, etc. Agassiz, J. L. 1844.5, 1874.2; André, W. 1784.1; Balkwill, F. H. 1875.1; Bicknell, E. 1872.1; Cocco, L. 1896.1; Forbes, H. O. 1879.1; Grube, A. E. 1878.2; Waldow, - 1911.1.

Miscellaneous early references to sharks' teeth, to be found in Pre-Linn. section. Geoffroy, C. 1741.1; Hérissant, F. 1753.1; Lister, M. 1674.1.

Dentition of Holocephali

The dental plate of Chimaroids (Callorhynchus) develops from a single enlarged flattened tooth according to Schauinsland, H. H. 1903.1 (p. 13, pl. 20).

General remarks on Chimaroid dentition. Hilgendorf, F. M. 1886.1.

Dental plates of Jurassic Chimaroids. Jaekel, O. M. 1901.1.

Fried

Dentition of Teleostomi Development and histology of teeth in Teleostei. Carlsson, A. 1894.1; mann, E. 1897.1; Ghigi, A. 1905.1,.2; Röse, C. 1897.1; Tomes, C. S. 1875.11900.1.

CHONDROSTEI and HOLOSTEI

The Acipenseridæ, according to Zograf, possess teeth while young. From the sterlet (Acipenser ruthenus), in which they are lost at the age of one year, a series extends to Psephurus gladius which retains them throughout life. Polydon likewise retains its teeth when mature.

References. Murray, A. 1871.1; Parker, W. K. 1881.1 (pl. 14, fig. 6); Pavlov, H. 1879.1; Peltzam, E. 1870.1; Zograf, N. Y. 1887.5, 1896.2. Lepidosteida.

Barkas, W. J. 1878.2. - Lepidosteus, analogies with labyrinthodonts. Wyman, J. 1844.1.

TEETH OF TELEOSTEI

Most systematic treatises include descriptions of the form and arrangement of the teeth. Numerous genera are based on such characters.

It is thus obviously out of question to attempt to give a complete list of references here. The following citations are to separate papers dealing with the dentition of the forms named.

Orthagoriscus.

Anarrhichas and Chatodon. André, W. 1784.1.- Scarida. Boas, J. E. 1879.2. Hilgendorf, F. M. 1893.1. Salmonida. Knox, R. 1855.1. Echeneis. Mummery, J. H. 1899.1; Murray, A. 1856.1. -Gymnodontes. Owen, R. 1839.4, 1840.3. Piabuca. Rowntree, W. S. 1906.1. Cyclopterus. Schmidt, B. 1913.1. - Esox. Tomes, C. S. 1878.1. - Mugil. Troschel, F. H. 1865.1. - Loricariide. Weyenbergh, H. - Anarrhichas. Wilson, A. 1879.1,.2. -Labrus. Wright, J. 1870.1. -Hydrocyon. Eastman, C. R. Add. 1917.1.

1875.2.

---

Although Coregonus wartmanni is toothless when adult, true rudimentary teeth occur in embryos of 1 cm. Walter, H. E. 1894.1.

Pharyngeal teeth of Teleostei

In certain Teleosts occur clusters of grinding teeth (pharyngeal teeth) capable of replacement by vertical succession.

The lower pharyngeals are usually composed of the paired ceratobranchial elements of the last (fifth) branchial arch which is reduced on each side to this element.

In the Cyprinidæ these grind against a callous pad above on the basi-occipital.

In the Labridæ they are opposed above by the upper pharyngeal teeth formed on the pharyngob.anchials of the posterior branchial arches.

For an excellent series, with photographs, of the pharyngeal teeth of many different species, see Shepherd, C. E. 1913.1.

Čerka

Pharyngeal teeth of Cyprinidæ. sov, P. G. 1903.1; Fack, 1897.1; ★ Gracianov, V. I. 1900.1; ★Hæmpel, O. 1907.2, 1908.2,.3; Hoppe, R. 1894.1; Jurine, L. 1821.2; Leonhardt, E. E. 1903.10; Molin, R. 1850.1; Nordmann, A. 1863.2.

Variability of pharyngeal teeth in Cyprinoid hybrids. Heincke, F. 1892.1.

Pharyngeal teeth of other fishes. Scarida. Boas, J. E. 1879.2. - Labrida, development. Prince, E. E. 1893.1. Orestias. Pellegrin, J. 1904.12. Gerrida. Sauvage, H. E. 1876.4. - Labrus. Wright, J. 1870.1.

Comparison of pharyngeals of Labrus from Italian Tertiary with those of recent Mediterranean species. Brunati, R. 1909.1.

Isolated fossil or sub-fossil pharyngeal teeth were earlier known ቢያ "Serpent's eyes." See Jussieu, A. Pre-Linn. 1725.1.

DERMAL SKELETON (OF FISHES) Comprising the morphology of the Exoskeleton, the Dermal Denticles, and the Scales.

The dermal fin-rays (dermotrichia), which constitute a part of the exoskeleton, are treated under Fins.

The dermal skeleton should properly include the "dermal or" membrane bones," but these are necessarily treated under Skull.

Scales are usually included under the general term "integument?' In the present work, however, under Integument, will be found references merely to the soft structures of the epidermis and the dermis.

44

In the Elasmobranchs, however, true placoid" denticles are present. These, more fully described below, consist of separate hollow cones of dentine capped by enamel." Each separate denticle, with growth, develops a more or less expanded basal portion, the "basal plate." In these fishes, true bone never develops.

In the higher bony-fishes (Teleostomes) and These in the Dipnoi the true scales occur. develop, usually as oval flattened structures, in pockets within the dermis and are at notime contributed to by the epidermis. They grow throughout life by the deposition of successive layers of bony substance.

Agassiz (J. L. 1833.2) divided fishes into the four groups, Placoidei, Ganoidei, Cycloidei, and Ctenoidei, based on the characters of their scales. That this arrangement was untenable, especially for the higher groups, was soon shown by the experience of one of his students, Prof. N. S. Shaler (Atlantic Monthly, Feb., 1909, p. 222), who discovered that one species of Pleuronectidæ "had cycloid scales on one side and ctenoid on the other." Likewise Peters (W. C. H. 1841.1) soon showed that the bonito (Sarda) possesses both cycloid and ctenoid scales. Agassiz (1850.6) decided that the scales of the bonito were intermediate and sometime later (1857.2) he definitely renounced his classification as being too artificial.

Except among palæontologists. where often the scales of certain fossil forms constitute the only remains preserved, the scales have not been much studied as an aid in the classification of fishes.

More recently Cockerell (T. D. 1909.41914.1) has assiduously studied and described the minute structure of the scales of many species in all groups, especially the Teleostomes, with the idea of defining the distinctive characters which may aid in the determination or identification of the species. The taxonomic value of " lepidology has however not since been extended.

Principal literature

Modern views on the morphology and derelopment of scales had their inception in the works cited under Williamson, W. C. 1849.1, and 1851.1.

The most comprehensive treatises in German, on the morphology of dermal (placoid) denticles and scales are ★Hertwig, O. 1876.1; and ★Klaatsch, H. 1890.1.

By far the most illuminative memoir (in English) on the development, structure, and phylogeny of denticles and scales is ★Goodrich, E. S. 1908.1.

Most of the previous views on the development and formation of scales are reviewed in English in Thomson, J. S. 1904.1.

As stated above, Cockerell (T. D. 1909.4– 1914.1, Add. 1914.1) has described the superficial characters of the scales of many genera and species of fishes. See especially ★Cockerell, T. D. 1911.5 and 1913.1.

Miscellaneous papers

Mechanical considerations of the evolution, or causes of the arrangement and shape of scales. Ryder, J. A. 1892.1; Woodward, A. S. 1893.5.

Older or miscellaneous and unimportant papers on the structure and manner of growth of the scales of fishes. Agassiz, J. L. 1840.2,.3,.4; Alessandrini, A. 1849.1;

Dermal Skeleton - Cont'd.

Baster, J. 1762.2..3; Benecke, B. 1882.1; Broussonet, P. M. 1787.1; Couch, J. 1868.2; Delsman, H. C. 1913.2; Hennah, T. H. 1873.1; Keene, J. H. 1879.2; Kuntzmann, J. H. 1829.1; ★Mandl, L. 1839.1-1840.1; Peters, W. C. 1841.1; Salbey, R. 1868.1-1870.1; Schmula, 1854.1; Steeg, G. 1857.1, 1861.1; Swaine, L. H. 1870.1; Anon. 173, 526, 599; Engström, A. Add. 1874.1; Huxley, T. H. Add. 1855.1.

The chief points in many of these older papers are reviewed in Baudelot, E. 1873.4; and in Thomson, J. S. 1904.1.

The scales of fishes were among the first objects examined by the early microscopists. The following references, partly referring to such observations, will be found in the Pre-Linn. section. Fabricius d'Aquapendente, J. 1618.1-1625.1; Hooke, R. 1665.1; Leeuwenhoek, A. 1686.1-1719.2.

EVOLUTION OF THE PROTECTIVE COVERING

Phylogeny of scales

Williamson (W. C. 1849.1, p. 466) attempted to outline the derivation of the Teleostome scale from the denticle of the Elasmobranch. He believed that by the fusion of the basal plates of a number of adjacent denticles a large composite denticle was formed. By the successive depression or insinking of this composite denticle (ultimately to entirely within the dermis), by the obliteration or disappearance of the separate tips of the denticles (aggregated to form the "cosmine" layer), and by the acquisition of a basal film of true bone which subsequently overlaps the outer surface, the true scale has been formed.

These views, with modifications, have been adopted by Hertwig, (O. 1876.1), by Wiedersheim (R. E. 1880.4), and by others. Klaatsch (H. 1890.1) believed that each scale represents a single denticle of which the basal plate has become greatly enlarged.

According to Goodrich (E. S. 1908.1), the views of Williamson are not without difficulties, the chief of which is the lack of intermediate stages between the denticle and the Cosmoid scale. While in general adhering to Williamson's views, Goodrich considers the elements of the dermal skeleton to be divisible into two distinct forms as follows:

(1) The dermal denticle of Elasmobranchs, of which the "basal plate" is simply the expanded lower portion of the external cone.

(2) The scale of Teleostomi-Dipnoi, divisible into Cosmine and Ganoid types. The essential part of the scale is the development in the dermis of a bony plate. The overlying denticles may have subsequently become fused to this plate and by aggregation or fusion may have become reduced to the cosmine layer. While the denticles have generally become lost since the appearance of scales, in certain forms (Lepidosteus, Polypterus and some Siluroids), minute denticles persist and become secondarily attached to the scales and to the dermal fin rays (lepidotrichia).

DENTICLES

The so-called Placoid Scale

of the Elasmobranchii

"True denticles are universally present in the living Elasmobranchs and their extinct allies.

"The placoid scale, or denticle, begins as a cone of dentine deposited by mesoblastic scleroblastic cells below the epidermis, in continuity with the basement membrane; a basal plate may be present in the form of a direct extension inwards of the cone, never as a separate element which becomes fused on to it secondarily; both the cone and the plate are composed of dentine or some allied substance, never of true bone; the cone may pierce the epidermis, when fully grown. Goodrich (1908.1, p. 753).

The outer surface of the denticle is covered with a hard, enamel-like layer of which the exact nature has been much disputed. As has been stated elsewhere (see Dentition), the identity of the teeth and denticles (dermal teeth") in Elasmobranchs has long been recognized. Röse (C. 1897.1) considered the enamel-like layer of the denticles and teeth to be a layer of dentine (vitrodentine). The arguments for and against its being enamel are summarized by Scupin (H. 1896.1). Although recognizing the primary part played by the mesoblastic dentine papilla in its formation, instead of its being wholly an epidermal secretion as is the true enamel of higher vertebrates, Tomes (C. S. 1898.1, p. 460) believes that this layer may be appropriately called enamel. It is not a dentine because there is no collagen matrix. For additional remarks on " Enamel," see under Dentition.

As pointed out by Steenstrup (J. J. 1861.2. .3), although probably incorrectly denied by Nardo (G. D. 1861.1), the denticles do not persist throughout the life of the shark and constantly increase in size as do the scales, but are shed individually to make place for others. They are subsequently replaced by the growth of newly developed denticles between the old

In the existing Chimæroids the skin is generally smooth but a few denticles are retained in rows along the head and back of young specimens. These are distinctly shark-like.

In addition, the males of all recent Chimaroids possess denticles, quoting from Dean (B. 1906.1, p. 117), "on the frontal clasping organ, on the mixipterygium, and on the anterior pelvic clasping organ. These denticles have a transparent almost glassy character. In the frontal clasping organ of Callorhynchus. they occur not only at the tip of the organ itself, but also proximalward and at the front and sides of the depression into which this clasping organ fits; but in the other genera, the denticles are limited only to the tip of this organ."

For a full discussion of the denticles in both recent and fossil members of this group, see ★Dean, B. 1906.1, pp. 114

118.

In Chimara and Hydrolagus, Cockerell records oval horseshoe-shaped denticles, which line and serve to keep open a mucous canal, lying below the dorsal denticles. Cockerell, T. D. 1913.3.

Certain fossil Chimaroids, Squaloraja and Myriacanthus, possess an enlarged frontal spine or tentaculum, evidently a frontal clasper, probably derived from denticles. Davies, W. 1872.1; Reis, O. M. 1895.2; Woodward, A. S. 1906.3.

Dermal plates of the Ostracodermi Concerning the exoskeleton of this early problematical group, we can do no better than quote from Goodrich (1908.1, p. 754), with necessary adaptations in reference numbers.

"The important researches of Traquair (R. H. 1899.2..3) have disclosed a most interesting series of Paleozoic fish in which it appears to be possible to trace clearly the evolution of the bony carapace of the Pteraspids from the simple placoid scales of Thelodus. The latter are broad and flattened denticles. Rohon

(J. V. 1889.2, 1893.1) and Röse (C. 1897.1) have described their finer structure. The pulp cavity is simple and there is no distinctly marked basal plate. Psammosteus is almost entirely covered with large plates (Traquair, R. H. 1899.3). Superficially these are studded with small denticles in every way similar to those of Thelodus. Below these denticles

is a thick plate of bone-like tissue, which, however, is devoid of bone cells.

"The denticles rest on the underlying plate, to which they become fixed, being fused to it here and there at their base.

"It is but a step from Psammosteus to Pteraspis, whose exoskeleton has been well described by Huxley (T. H. 1858.2) and Lankester (E. R. 1864.2, 1865.1)..

"Thus it appears that the shields of the Heterostraci, and also the scales and dermal fin-rays, have all been evolved by the combination of a covering of separate isolated denticles and an underlying plate, and the theory of Williamson is confirmed in a most remarkable manner by Traquair.'

Professor Goodrich, having read proofs of this section, suggests:

"In the same way, within the Osteostraci, we can trace the development of a complete armour of scales and plates from primitive separate denticles which become fused on to underlying plates of true bone with bone cells."

THE TRUE SCALES

As stated above, Goodrich divides the scales previously known as ganoid" into two types, the cosmoid of which the scale of Megalichthys (Osteolepidæ), well described by Williamson (W. C. 1849.1), is an example and the ganoid which is further divisible into two varieties.

A cosmoid scale grows only by the addition of new cosmine at the edge and of isopedine (bone) on the lower surface. No living fish has a cosmoid scale. Among extinct forms it occurs only in the extinct Crossopterygii and in the Dipterida.

The Ganoid scale

The "ganoid scale, as restricted by Goodrich, is found in all recent "ganoids" (Goodrich's Actinopterygii). It does not occur in the Teleosts.

These scales are generally thick and rhomboid. They grow by the deposition of concentric layers over the whole surface, the oldest part of the scale being at the center. These layers on the lower surface are bony or fibrous; on the outer surface they are of the enamel-like substance called, in the case of Lepidosteus,

ganoin by Williamson (W. C. 1849.1, p. 438). As shown by Hertwig and by Nickerson, the ganoin is entirely mesoblastic in origin and has no relation to enamel.

Goodrich (1908.1, p. 757) divides the ganoid scale into two distinct varieties, the "Palæoniscoid" type and the "Lepidosteoid ' type. The latter is pierced by characteristic tubules, found also throughout the bony skeleton of living and fossil Amioidei and Lepidosteoidei (Goodrich, 1913.2).

Histological structure of “ganoid" scales in numerous fossil genera. Klaatsch, H. 1890.1; Scupin, H. 1896.1.

Structure of the scales in Polypterus. Hertwig, O. 1876.1; Leydig, F. 1854.1; Reissner, E. 1859.1; Scupin, H. 1896.1.

The presence of small scales in the integument of Polyodon is recorded in Collinge, W. E. 1895.1.

In Acipenser, the scales are relatively few, large plates composed of concentric layers of bone, entirely lacking the ganoin layer. Agassiz, J. L. 1856.1; ★Hertwig, O. 1876.1 (vol. ii); Kosmak, G. W.

1895.1.

Structure and development of the scales in Lepidosteus. Agassiz, J. L. 1856.1; Hertwig, O. 1876.1 (vol. v); Nickerson, W. S. 1893.1; Reissner, E. 1859.1.

In Amia the scales are thin and the ganoin vestigial. Unimportant references are Green, J. 1862.1; and Mackintosh, H. W. 1878.1.

Scales of Teleosts

The scales of Teleosts, although presenting great variations in form and structure, are in general thin and flexible and develop within dermal pouches. Their posterior edges usually slightly overlap the following scale.

Scales increase in size by the deposition of fine excentric rings entirely around the outer margin. These rings have been termed growthrings (Zuwachsringe) and the fine lines limiting the growth rings are the lines of growth (Thomson, J. S. 1902.1).

During summer, with its favorable conditions for growth, the scale rapidly increases in size and the lines of growth are relatively far apart, but during the following winter, the growthrings become fewer and narrower and the lines of growth consequently become crowded together. This variation produces a distinct appearance, the annual ring roughly comparable to the rings in many trees. These annual rings, as first demonstrated by Hoffbauer (C. 1898.1 et seq.), supply us with an index to the age of the fish.

The growth and annual rings have been much studied in recent years in connection with the determination of the age and growth of fishes. This constitutes a separate subject with an extensive literature, for which see "Recent methods in age determination under Growth and age.

The scales grow continuously throughout life. If broken off or otherwise injured, they possess the power of regeneration (Mohr, E. 1914.1; Scott, W. 1911.1). Ryder (J. A. 1884.4) records that a carp, in which a scale had been detached, had, after five months, replaced the lost scale with one of the same size but thinner and also lighter in color. The condition of the growth-rings was not determined.

Cycloid and ctenoid scales

As indicated above (under Introductory remarks), the differences between cycloid and