502 POLYPTERINI [CH. 



of members of both families, from Polyodontidae in America and 

 from Acipenseridae in Russia. 



The Acipenseridae are, like Salmon, anadromous fish, spawning 

 in fresh water, but, with the exception of one or two species, seeking 

 their living in the sea. The Polyodontidae, so far as is known, are 

 fresh-water. Acipenser is found occasionally off the British coast, 

 but abounds in the rivers of Russia and the Black Sea. Polyodon is 

 a denizen of the Mississippi. 



To the order Chondrostei belong a large number of fossil fish, and 

 as we recede backwards in geological times the peculiar features of 

 the Sturgeon gradually fade out. The rostrum in older forms is 

 shorter, a premaxilla as well as a maxilla is present, and both bear 

 teeth (as the maxilla does in the young Polyodon); the body is 

 clothed with shining " ganoid " scales, like those of Lepidosteus and 

 Amia, and the whole animal looks very like an ordinary fish, but is 

 destitute of an ossified backbone and, so far as is known, of car- 

 tilage bone in the skull. 



These extinct fish are termed "Palaeoniscids," and in this case 

 their evolution into Sturgeons has been a degeneration, not an 

 advance, in structure. 



Order V. Polypterini. 



The Polypterini include two genera, Polypterus and Cala- 

 moichthys, both confined to the rivers of Africa. This order agrees 

 with the three preceding in the archaic features of conus, optic, 

 chiasma, and spiral valve, but it agrees very nearly with Teleostei 

 in the character of the male organs. As in that order there is a 

 sterile posterior prolongation of the testes consisting of a central 

 duct with a network of sterile tubes around it, and this portion of 

 the testes, as in the young Teleostei, communicates with the hinder 

 portion of the archinephric duct by a single tube which may be 

 looked on as a single posterior vas efferens. In the female the 

 oviduct is short and opens by a wide funnel into the coelom. 

 The Polypterini further agree with the Teleostei in the character 

 of the vertebral column, which consists of well ossified amphicoelous 

 vertebrae bearing two sets of ribs, viz. dorsal ribs, which correspond 

 to the "epipleurals" of Teleostei and the ribs of Chondrichthyes, 

 and ventral ribs, which correspond to the ribs of Teleostei and all 

 other Osteichthyes. 



The peculiarities of Polypterini are to be found in their fins 



