400 



AMERICAN MUSEUM OF NATURAL HISTORY 



Dermal Skeleton — Cont'd. 

 Raster, 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. 1S57.1, 1861.1; Swaine, 

 L.H. 1870.1; Anon. 173, 526, 599; Eng- 

 strbm, 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 micro- 

 scopists. The following references, partly 

 referring to such observations, will be 

 found in the Pre-Linn. section. Fabri- 

 eius d'Aquapendente, J. 1618.1—1625.1; 

 Hooke, R. 1665.1; Leeuwenhoek, A. 

 1686.1-1719.2. 



EVOLUTION OF THE PEOTECTIVE 

 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 suc- 

 cessive 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 ac- 

 quisition 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 Wieder- 

 sheim (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 be- 

 come 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 William- 

 son'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 essen- 

 tial 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 (Lepi- 

 dosteus, Polypterus and some Siluroids) , minute 

 denticles persist and become secondarily at- 

 tached to the scales and to the dermal fin rays 

 (lepidotrichia). 



DENTICLES 



The so-called Flacoid Scale 



of the Elasmobrauchii 



" 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 

 scleroblastio cells below the epidermis, m con- 

 tinuity with the basement membrane; a basal 

 plate may be present in the form of a direct 

 extension inwards of the cone, never as a sepa- 

 rate 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. 763). 



The outer surface of the denticle is covered 

 with a hard, enamel-Uke 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 rec- 

 ognized. Rose (C. 1897.1) considered the 

 enamel-like layer of the denticles and teeth to 

 be a layer of dentine (vitrodentine) . The argu- 

 ments 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 secre- 

 tion 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 con- 

 stantly 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 

 ones. 



Plagiostomi 



Papers relating to the development, 

 structure, and histology of the denticles or 

 "placoid scales" of sharks and rays. Benda, 

 C. 1881.1; Brackel, G. 1858.1; Green, J. 

 1859.2; Hannover, A. 1868.1; •Hert- 

 wig, O. 1874.1; •Klaatsch, H. 1890.1; 

 •Kwietniewski, C. 1905.1; Meyer, P. 

 1886.1; •Burckhardt, C. R. Add. 



1902.1. 



For excellent figures of the denticles of 

 certain sharks and rays, see Garman, S. 

 1913.1, and Radcllffe, L. 1916.1. 



The exoskeleton of Heterodontus fran- 

 cisci. Daniel, J. F. Add. 1914.1. 



Holocephali 



In the existing Chimseroids the skin is gen- 

 erally 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 Chimae- 

 roids possess denticles, quoting from Dean 

 (B. 1906.1, p. 117), "on the frontal clasping 

 organ, on the mixipterygium, and on the an- 

 terior 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- 



llo. 



