396 



MANUAL OF ZOOLOGY 



SECT. 



may be so large and strong as to form a rigid armour 

 Sometimes there is an armour formed of stout bony plates, 

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



FIG. 222. A, ctenoid scale ; B, ganoid scales. (After Giinther.) 



exoskeleton takes the form of minute spines like the 

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

 spines; lastly in Polypterus (Fig. 223) and Lepidosteus are 

 found rhomboid or ganoid scales in the form of rhomboidal 





FIG. 223. Polypterus birchir. A, entire animal ; B, ventral 

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

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

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

 tral fin. (After Cuvier.) 



plates of bone covered externally by a layer of enamel-like 



ganoin and joined together by pegs and sockets (Fig. 222, B). 



In the Sturgeon the spinal column consists of a persistent 



notochord with cartilaginous arches ; in the rest bony 



