148 



ANATOMY OF VERTEBRATES. 



364 



Ganoid scales, Amblyptirus. 



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 arc 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 

 365 • calcareous granules, ib. e ; 



and the internal layer of 

 lamina} of structureless car- 

 tilage, ib. c. 



In the Tunny ( Thjnnus 

 vulgaris) the scales are com- 

 posed of fine, partially ossi- 

 fied, laminae, between which 

 are elongated ' lacunas ' 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 

 r^thin. diameter, and having 



8OU0 ' C3 



numerous lacunae 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 

 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 ' lepidine ' tubules in 



1 ' Corpuscles of Mandl,' ccciii.: ' Lenticular bodies' of Williamson, ccxen. 

 2 nnxem. vol. ii. p. "0. s v. p. 14. ' ccxci. p. 439. 



Placoid scale a, b, uat. size ; c, magn. section, xxn. 



