TEGUMENTS OF FISHES. 549 



the parts of the scale to which they are limited. The peripheral 

 surface of the scale is coated by a layer of hard transparent lami- 

 nated substance, ib. b l (' email/ Ag. ; 'ganoin,' ' Wilk.' 2 ). In Lepi- 

 dosteus and Potypterus the ganoin is usually confined to two-thirds 

 of the outer surface of the scale. In both existing Ganoids the 

 scale is perforated vertically by a few larger 

 tubes, analogous to the ' vascular canals ' of ___ s66 

 bone, and conveying blood from the derm to 

 the delicate layer of membrane extending from 

 the scale-pit, fig. 360, f, upon the ganoin. 

 In the scales of extinct Ganoidei a portion 

 of the deeper layer of ganoin traversed by 

 minute branching tubuli, and devoid of 

 lacuna), has received the name of e kosmin.' 3 structure of ganoid scales. 



CQXG.IU 



This tissue constitutes the chief part of the 



tooth-shaped bodies of the ' shagreen ' of the Dog-fish and many 

 other Placoids ; in which it cannot be histologically distin- 

 guished from hard dentine. 4 In fig. 365 are given a side view, A, 

 and upper view, B, of one of the ( placoid scales,' or spiny tubercles 

 of the Thornback (Raia clavata) : C is a magnified section of the 

 scale. The substance of the spine consists of superimposed conical 

 lamellae, having, in the -Thornback, a widely open pulp-cavity, 

 ib. p, from which proceed vascular canals, resolving into plasmatic 

 tubes, radiating and ramifying through the substance as in ordi- 

 nary teeth. The base of the spine-bearing scale, ib. Z>, is imbedded 

 in the derm ; and, as the ' haversian canals ' of the jaw pass into 

 the ( medullary canals ' of the teeth thereto anchylosed, so do the 

 capillaries of the derm pass directly into the ( vascular ' canals 

 and pulp-cavity of the dermal dentine in all the various forms of 

 placoid scales, many of which have a coating of true e ganoin ' 

 over the fine-tubed dentine or ' kosmin.' Other modifications of 

 the dermoskeleton, such as the placoganoid and acaiithoganoid, 

 are noticed at pp. 193-198, and illustrated in figs. 124-127. 



The calcification of scales, as of teeth, fig. 242, and bone, fig. 

 15, takes place in layers of the organic basis successively formed: 

 but the primitive lamellate condition is most conspicuous in fish- 

 scales. The idea of excretion, or the throwing out of such layers 

 from a secreting surface, is, however, as inadequate to represent 

 the facts of the formation and structure of the exoskeleton as of 

 the endoskeleton of fishes. 5 



1 XXTI. vol. i. p. 74. - ccxci. p. 438. 3 Ib. p. 444. 4 v. p. 14. 



5 The microscopical observations on the structures of recent and fossil teeth, with 

 incidental notices of corresponding organisation in ossified scales, communicated by 



