SUBJECT IXD EX — MORPHOLOGICAL SECTION 



401 



\In ChimcEra and Hydrolagus, Cockerell 

 records oval horseskoeshaped de^iticles, 

 which line and serve to keep open a mucous 

 canal, lying below the dorsal denticles, 

 Cockerell, T. D. 1913.3. 



Certain fossil Chimceroids, Squaloraja 

 and Myriacanthus, possess on enlarged 

 frontal spine or tentaciihiTn, evidently a 

 frontal cUisper, probably derived from den- 

 ticles. Davies, W. 1872.1; Reis, O. M. 

 1S95.2; Woodward, A. S. 1906.3. 



Dermal plates of the Ostracoderzui 



Concerning the exoskeleton of this early 

 problematical group, we can do no better than 

 quote from Goodrich (190S.1, p. 754), with 

 necessary adaptations in reference numbers. 



" The important researches of Traquair 

 (R. H. 1899 .2,.3) have disclosed a most interest- 

 ing series of Palaeozoic 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 Thelodtts. The latter 

 are broad and flattened denticles. . . . Rohon 

 (J. V. 1889.2, 1893.1) and Rose (C. 1897.1) 

 have described their finer stmcture. The pulp 

 caWty is simple and there is no distinctly marked 

 basal plate. . . . Psammosteus is almost en- 

 tirely covered with large plates (Traquair, R. 

 H. 1S99.3). SuperfieiaUy 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, how- 

 ever, is devoid of bone cells. 



" The denticles . . . rest on the underljdng 

 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 Lan- 

 kester (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 combina- 

 tion of a covering of separate isolated denticles 

 and an underlj-ing 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 Osteostrad, 

 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 TBUE SCALES 



As stated above, Goodrich divides the scales 

 previously known as " ganoid " into two types, 

 the cosmoid of which the scale of Megalichthys 

 (Osteolepidse) , 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. ^ Xo Uving fish has 

 a cosmoid scale. Among extinct forms it occurs 

 only in the extinct Crossopterygii and in the 

 Dipteridae. 



The Ganoid scale 

 The *' ganoid " scale, as restricted by Good- 

 rich, is found in all recent " ganoids " (Good- 

 rich's Actinopterygii). It does not occur in the 

 Teleosts. 



These scales are generally thick and rhom- 

 boid. They grow by the deposition of concen- 

 tric layers over the whole surface, the oldest 

 f)art of the scale being at the center. These 

 ayers on the lower surface are bony or fibrous; 

 on the outer surface they are of the enamel-Uke 

 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 " PalsBO- 

 niscoid " type and the " Lepidosteoid " type. 

 The latter is pierced by characteristic tubiiles, 

 found also tliroughout the bony skeleton of 

 hving 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 s?nall scales in the in- 

 tegument of Polyodon is recorded in Col- 

 linge, 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; -A-Hertwig, 

 O. 1876.1 (vol. ii); Kosmak, G. W. 

 1895.1. 



Structure and development of the scales in 

 Lepidosteus. Agassiz, J. L. 1856.1; Hert- 

 wig, 0. 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 growth- 

 rings (Zuwachsringe) and the fine lines limiting 

 the growth rings are the lines of growth (Thom- 

 son, J. S. 1902.1). 



During siunmer, with its favorable conditions 

 for growth, the scale rapidly increases in size 

 and the lines of growth arfe relatively far apart, 

 but during the following winter, the growth- 

 rings become fewer and narrower and the lines 

 of growth consequently become crowded to- 

 gether. This variation produces a distinct ap- 

 pearance, 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 vn.th. an 

 extensive literature, for which see ' ' JElecent 

 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. 1S84.4) 

 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 re- 

 marks), the differences between cycloid and 



