IV. VERTEBRATA: PISCES, GAN01DEI. 573 



membrane bones lie on the shoulder girdle, on the roof and floor 

 of the skull (parasphenoid) ; the horny threads of the fins are bony 

 rays. In general the skeleton ranges between two extremes an 

 extremely primitive cartilaginous condition with persistent noto- 

 chord, and one with a more than ordinary degree of ossification. 

 It is important for the systematist to find characters in all ganoids 

 which occur only in the group. The ganoid scales, used by Agassiz, 

 are not sufficient, since the sturgeon has bony plates free from 

 ganoin, while the paddle bill (Polyodon*) has almost no dermal 

 skeleton, and Amia has cycloid scales. Most recent and fossil 

 forms possess fulcra, bony plates with forked ends lying shingle- 

 like in front of the fins (fig. 10, B), but these are not universal, 

 and are absent, e.g., in Amia and Polypterus (fig. 10, A and C). 

 The group is largely American. The few recent ganoids fall into 

 three distinct groups. 



Order I. Crossopterygii. 



These are largely extinct, but two genera persisting to-day. The tails 

 are diphycercal or heterocercal; the pectoral fins have the basal portion 

 scaled ; broad gular plates beneath the jaws in place of branchiostegals; the 

 skeleton well ossified. Polypterus and Calamoichthys from Africa. The 

 order was probably ancestral to the Amphibia. 



Order II. Chondrostei. 



These forms resemble the sharks externally in the heterocercal tail, 

 spiracle, ventral position of the mouth; internally in the cartilaginous 

 skull and (except Polyodori) in the pterygoquadrate serving as upper jaw. 

 In the vertebral column they are more primitive than most selachians, 

 since centra are lacking, the neural and haemal arches and the intercalaria 

 resting direct on the notochordal sheath (Sg. 556). ACIPENSERIIME, with 



FIG. 601. Acipenser sturio* common sturgeon. (After Goode.) 

 large bony dermal plates. Acipenser,* sturgeon. The swim bladder 

 furnishes isinglass, the ovaries make caviare. POLYODONTIDJB, with 

 naked skin and long paddle-like snout, toothed maxillaries present. 

 Polyodon* paddle fish. 



Order III. Holostei. 



In these the skull is ossified as in teleosts; maxillary and premaxillary 

 bones are present, the pterygoquadrates reduced and not meeting in front, 

 and the mouth terminal. The body may be covered either with ganoid or 



