XII 



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 Giinther.) 



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. 



FIG. 239. Polypterus birchir. A, entire animal ; B, ventral 

 view of throat, an, anus; br. tit, branchiostegal membrane; 

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



elvic fin; v. 



., ., 



na, nostril: pet./, pectoral fin; pv.f, pel 

 tral fin. (After Cuvier.) 



.f, ven- 



In the sturgeon the spinal column consists of a persistent 

 notochord with cartilaginous arches ; in the rest bony verte- 

 brae are present, the centres of which are nearly always 

 bi-concave. 



