190 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- 
Fic. 105.—Cyeloid scale of Salmo fario. Fic. 106.—A, Ctenoid scale; B, “*Ganoid”’ 
a, Anterior portion covered by overlap scale. (After Giinther ; from Parker and 
of preceding scales ; 6, free portion Haswell.) 
covered only by pigmented epidermis. 
(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, Monacanthus, Tria- 
canthus).” 
Amongst some of the usually scaleless 
Siluroid Fishes the scales assume a _ very 
peculiar structure. In Hypostoma® (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- 
a ; velopment of the scales of Lepidosteus, when the 
Fic. 107.—Centriscus sco- . ; 
lopax. A,Scale from independently formed and evanescent spines 
the orbital region, x have not yet fused with the basal plates. 
50; B, scale from the ; ‘ 
base of the pectoral In other Teleosts, as in the Agonidae 
ey (ron and some Trighdae, the body is completely 
cuirassed with large keeled bony plates. 
The singular appearance of many of the Plectognathi is largely 
' O. Hertwig, Morph. Jahrb. vii. p. 7. 2 Ibid. vii. p. 29. 3 [bid. ii. p. 334. 
