548 



ANATOMY OF VERTEBRATES. 



364 



Ganoid scales, Amblypterus. 



365 



termed ( placoid,' fig. 365, < ganoid,' fig. 364, < cycloid,' fig. 362, 

 ' ctenoid,' fig. 363. In the first and second kinds bone-earth predo- 

 minates, and the scales are as hard as teeth ; in the two latter kinds 



the earth is in less quantity, so that 

 the scale is flexible : it is rarely want- 

 ing. The kinds of scale graduate 

 into each other. Most flexible scales 

 present two structures: one next the 

 derm is composed of gristly lamina?, 

 usually firm and elastic, fig. 360, c ; 

 the superficial portion, ib. d, is 

 laminated and hardened by interla- 

 mellar calcareous granules. 1 In the posterior or exposed part 

 of the scale of a Carp there is a peripheral osseous layer, deve- 

 loping the outer markings or projections of the scale, fig. 360, d\ a 



middle laminated layer, with 

 calcareous granules, ib. e ; 

 and the internal layer of 

 lamina? of structureless car- 

 tilage, ib. c. 



In the Tunny ( Thynnus 

 vulgaris) the scales are com- 

 posed of fine, partially ossi- 

 fied, lamina?, between which 

 are elongated ( lacuna? ' or 

 bone-cells: the scale is cancel- 

 lous at its middle part. 2 In Le- 

 pidosteus the scale is thicker, 

 is composed of very thin ossi- 

 fied layers, fig. 366,, perfo- 

 rated by vertical tubes about 

 gljjjjjth in. diameter, and having 

 numerous lacuna? in their 

 interspaces, 3 the radiating 

 canals of which communicate 

 with a more minute series of vertical branched tubules, called ( Le- 

 pidine ' by their describer. 4 These, in most ganoid fishes, have a 

 less general distribution through the scale than the larger and less 



o ~ 



branched series, which, from their analogy to dentinal tubes, I have 

 called ( plasmatic,' conceiving them to relate to the nutrition and 

 vitality of the scale, as doubtless also do the l lepidine ' tubules in 



Placoid scale A, B, nat. size ; 0, magn. section, 



1 ' Corpuscles of Mandl,' cccm. : 'Lenticular bodies' of Williamson, ccxcn. 

 2 ooxciii. vol. ii. p. 70. 3 v. p. 14. 4 ccxci. p. 439. 



