XIII 



PHYLUM CHORDATA 



203 



abdominal. A spiral valve and a conus arteriosus are present, and 

 the optic nerves form a chiasma. 



The only existing members of this order are Polypterus lichir 

 (Fig. 823), from the Upper Nile, and Calamoichthys calabaricus 

 from Old Calabar. 



ORDER 2. CHONDROSTEI. 



Teleostomi in which the paired fins have no basal lobe, but their 

 whole free portion is supported by dermal rays. There are few 



pctj 



FIG. 824. Acipenser rutbenus (Sturgeon), b. barbels ; c. /. caudal fin ; d. /. dorsal fin 

 pet. f. pectoral fin ; pv. f. pelvic fin ; sc. scutes ; v. f. ventral fin. (After Cuvier.) 



cartilage bones in the skull, and the primary shoulder-girdle is 

 unossified. The vertebral column consists of a persistent notochord 

 with cartilaginous arches, and its anterior end is fused with the 

 cranium. Branchiostegal rays are few or absent. The tail is 

 heterocercal. The pelvic fins are abdominal. A spiral valve, 

 conus arteriosus, and optic chiasma are present. 



This order includes the Sturgeons (Acipenser and Scaphirhynchus, 

 Fig. 824), found in the rivers of Europe, Asia, and North America ; 

 the curious spoon-billed Polyodon, from the Mississippi ; and 

 Psephurus from the rivers of China. 



ORDER 3. HOLOSTEI. 



Teleostomi in which the paired fins have no basal lobe. The 

 chondrocranium is well ossified by cartilage bones and invested by 



FIG. 825. Iiepidosteus platystomtis (Bony Pike), c. . caudal fin; (?../. dorsal fin; 

 n. fulcra; 1. I. lateral line; pct.f. pectoral fin; pi: f. pelvic fin; v. /. ventral fin. (After 

 Cuvier.) 



membrane bones : branchiostegal rays are present. The vertebral 

 column consists of bony vertebrae, and the tail is heterocercal or 



