548 



ANATOMY OF VERTEBnATES. 



364 



Ganoid scales, AMblppkrus. 



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

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

 minates, and tlie 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- 

 inw. The kinds of scale graduate 

 into each other. Most flexible scales 

 present two structures : one next the 

 derm is composed of gristly laminte, 

 usually firm and elastic, fig. 360, c ; 

 the superficial portion, ib. d, is 

 laminated and hardened by interla- 

 mellar calcareous granules." 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, c?; a 



middle laminated layer, with 

 365 calcareous granules, ib. e ; 



and the internal layer of 

 laminse of structureless car- 

 tilage, ib. c. 



In the Tunny (Thi/imus 

 vulgaris) the scales are com- 

 posed of fine, partially ossi- 

 fied, laminre, between which 

 are elongated ' lacunas ' or 

 bone-cells: thescaleis cancel- 

 lous atitsmiddlepart.^ In Z-e- 

 pidosteus the scale is thicker, 

 is composed of very thin ossi- 

 fied layers, fig. 3 6 6, a, perfo- 

 rated by vertical tubes about 

 -!-thin. diameter, and having 



tut in ^ o 



numerous lacunaj in their 

 interspaces,' the radiating 

 canals of which communicate 

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

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

 less general distribu.tion through the scale thair the larger and less 

 branched scries, 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 ' tidnilcs in 



' ' Corpuscles of MaiuU,' ccciii.: ' Ijcnticiihir liojics' of Williamson, ccxcii. 

 ^ ccxoiii. vol. ii. p. 70. = V. p. It. 1 coxin. p, 4rty. 



Phiciiid scale A, n, Eat. size ; c, magn. section, xxil. 



