PHYLUM CHORDATA 



399 



Sometimes there is an armour formed of stout bony plates, 

 or scutes, while in other cases, as in the " file-fishes," the 

 exoskeleton takes the form of minute spines like the 



Fig. 238. — ■ A, ctenoid scale ; B, ganoid scale. (After Gunther.) 



shagreen of sharks, or as in many globe-fishes of long bony 

 spines ; lastly, in Polypterus (Fig. 239) and Lepidosteus are 

 found rhomboid or ganoid scales in the form of rhomboidal 

 plates of bone covered externally by a layer of enamel or 

 ganoin and joined together by pegs and sockets. 



^ 



br.m pot^f 



pvf 



Fig. 239. — Polypterus birchir. A, entire animal; B, ventral 

 view of throat, an, anus; dr. 7«,branchiostegal membrane; 

 c.f, caudal fin; d.f, dorsal finlets; jug. pi, jugular plates; 

 na, nostril; pet./, pectoral fin; pv.f, pelvic fin; v.f, ven- 

 tral fin. (After Cuvier.) 



B 



Ju-ff.pL 



In the sturgeon the spinal column consists of a persistent 

 notochord with cartilaginous arches ; in the rest bony verte- 

 bra are present, the centres of which are nearly always 

 bi-concave. 



