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 the basal plate (Fig. 107). Some Mal- 



FIG. 105. Cycloid scale of Salmo fario. 

 a, Anterior portion covered by overlap 

 of preceding scales ; 6, free portion 

 covered only by pigmented epidermis. 

 (From Parker and Haswell.) 



FIG. 106. A, Ctenoid scale ; B, "Ganoid " 

 scale. (After Giinther ; from Parker 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, Mbnacanthus, Tria- 

 canihus}? 



Amongst some of the usually scaleless 

 Siluroid Fishes the scales assume a very 

 peculiar structure. In Hypostoma 3 (Pleco- 

 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 ofLepidosteus, when the 

 FIG. 107. Centriscus sco- .-,-,.,-, 



lopax. A, Scale from independently formed and evanescent spines 



the orbital region, x h ave no t vet f usec l w jth the basal plates. 

 50 ; B, scale from the 



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



m aud some Tri 8 lidae > the bodv is completely 

 cuirassed with large keeled bony plates. 

 The singular appearance of many of the Plectognathi is largely 

 1 0. Hertwig, Morph. Jahrb. vii. p. 7. 2 Ibid. vii. p. 29. 3 Ibid. ii. p. 334. 



