I go 



FISHES 



CHAP. 



of a rhombic basal plate, produced into a curved, backwardly- 

 inclined spine, the axis of which contains a pulp-cavity opening 

 on the inner surface of tlie basal plate (Fig. 107). Some Mal- 



FiG. 105. — Cycloid scale of Scdmo fario. Fio. 106. — A, Ctenoid scale; B, "Ganoid" 



a. Anterior portion covered by overlap scale. (After Gunther ; from Parker and 



of preceding scales ; b, free portion Haswell.) 

 covered only by pigmented epidermis. 

 ( From Parlier and Haswell. ) 



thidae (e.g. Malthe ^) have similar scales, but with round basal 

 plates and solid spines (Fig. 108, B). Similar scales (Fig. 109), 

 sometimes rhombic in shape, with one or more spines, which may 

 be simple or branched, are also found in the 

 Sclerodermi (e.g. Balistes, Monacantluis, Tria- 

 canthus)? 



Amongst some of the usually scaleless 

 Siluroid Fishes the scales assume a very 

 peculiar structure. In Hypostoma^ (Fleco- 

 stomus) the sides and back of the Fish are 

 covered by large bony plates, but on the 

 under surface and on the fins these are 

 replaced by much smaller ones. Both kinds, 

 however, carry numerous small movable spines 

 implanted in sockets (Fig. 110), a fact which 

 suggests comparison with a stage in the de- 

 velopment of the scales oiZepidosteus, when the 

 lopax. A, Scale from independently formed and evanescent spines 



soVBl'^ri^Zmthe ^^^^ ^°* ^^^ ^""'^^ ^^^h the_ basal plates. 



base of the pectoral In other Tcleosts, as in the Agonidae 



He'rtwigT' *^'™ ^""^ some Triglidac, the body is completely 



cuirassed with large keeled bony plates. 



The singular appearance of many of the Plectognathi is largely 



^ 0. Hertwig, Morph. Jalirh. vii. p. 7. ' Ibid. vii. p. 29. ^ Ibid. ii. p. 334. 



Fig. 107. — Centriscus sco- 



