Ichthyopsida 



429 



by a thick layer of peculiar and very hard calcareous substance, 

 ganoin, resembling enamel but of dermal origin. Spiracles open; 

 four pairs of gill-clefts. Air-bladder ventral to digestive tube and 

 permanently open into pharynx, therefore virtually a lung. 



A 



Fir.. 332. Skeletons of tails of fishes. 

 (A) Sturgeon (Acipenser). (B) (iar pike 

 (Lepidosteus). (C) Tarpon, a Eeleost. 

 (Courtesy, American Museum of Natural 

 History, New York.) 



This order includes only two living genera, Polypterus (Fig. 

 3336) and Calamoichthys, occurring only in Africa. Best known 

 is Polypterus, found in the Nile and Congo rivers, the generic name 

 referring to the numerous small dorsal fins. 



Order Holostei 



Skeleton well ossified except for more or less cartilage persistent 

 in cranial part of adult skull. Bony vertebrae with centra. Notochord 

 not persistent, or only remnants of it, in adult. Tail-fin hetero- 

 cercal (Lepidosteus: Fig. 332B) or approximately homocercal (Amia: 

 Fig. 333D). Body completely covered by scales (Fig. 328) of the 

 heavy ganoin-covered type (Lepidosteus) or merely thin plates of bone 

 (Amia). Embryonic spiracles closed in adult; five pairs of gill-clefts. 

 Air-bladder dorsal to digestive tube and permanently open. Spiral 

 valve very weakly developed. 



