218 



ZOOLOGY 



arteriosus is sometimes present, sometimes absent ; when it is absent 

 there is a large bulbus aortse formed as a dilatation of the ventral 

 aorta. The prosencephalon has usually a non-nervous roof; the 

 optic nerves either form a chiasma or simply decussate. The ova 

 are small ; the gonoducts are either continuous with the gonads, 

 or open anteriorly into the coelome, or are absent : in the latter 

 case the sexual products pass out by genital pores ; true abdominal 

 pores may be present in addition. Segmentation of the egg is 

 either entire or discoidal ; development is sometimes accompained 

 by a metamorphosis. 



The Teleostomi are classified as follows : — 



Order 1. — Crossopt'erygii. 



Teleostomi in which the pectoral fin consists of a rounded basal 

 lobe supported by endoskeletal structures and fringed by dermal 



br-.m pri^f 



p-'tr 



Fio. S72.— Polypterus bichir. A, entire animal; B, 

 ventral view of throat, an. anus ; hr. vi.' branchiostegal 

 membrane ; c. f. caudal fin ; d. f. dorsal finlets ; jug. pi. 

 jugTilar plates ; ?ia. nostril ; pc^. /. pectoral fin ; pij. /. 

 pelvic fill ; V. /. ventral fin. (After Cuvier.) 



J"^ffP' 



rays. There are no branchiostegal rays. The vertebral column is 

 well ossified, and the caudal fin is diphycercal. The pelvic fins are 

 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 several species of 

 Polypterus (Fig. 872) from the Congo and Upper Nile, and 

 CalamicMhys calaharicus 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 

 replacing 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. 



