202 



ZOOLOGY 



SECT, 



jaws are connected with the cranium through the intermediation 

 of a hyomandibular, which is probably not homologous with the 

 similarly named element of Elasmobranchs. The dermal fin- 

 rays are formed of membrane bone, and are supported by 

 pterygiophores which may be either cartilaginous or bony, but 

 which always show a great reduction in number as compared 

 with the homologous structures in Elasmobranchs. The primary 

 shoulder-girdle is complicated by the addition of membrane 

 bones, of which a large clavicle is the most constant. The 

 pelvic girdle is vestigial or absent. The pelvic fins usually 

 undergo a forward displacement, their position being either abdo- 

 minal, i.e. between the anus and the pectoral region, or thoracic, i.e.. 

 in the pectoral region, or jugular, i.e. under the throat. A dermal 

 exoskeleton is usually present. The intestine may or may not 

 have a spiral valve : the anus is distinct from, and placed in front 

 of, the urinary and genital apertures. The gills are covered by an 

 operculum supported by membrane bones, and the interbranchial 

 septa are reduced or absent, so that the gill-filaments are partially 

 or wholly free ; the hyoidean gill is reduced or absent. The conus 

 arteriosus is sometimes present, sometimes absent ; when absent 

 there is a large bulbu s aortse formed as a dilatation of the ventral 

 aorta. The prosencephalon has 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 ccelome, 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 accompanied by a metamorphosis. 



The Teleostomi are classified as follows :- 



ORDER 1. CROSSOPTERYGII. 



Teleostomi in which the pectoral fin consists of a rounded basal 

 lobe supported by endoskeletal structures and fringed by dermal 



no. 



bf. m, 



FIG. 823. Polypterus bichir. A, entire animal ; B, 

 ventral view of throat, "/'.anus; In: nt. branohiostegal 

 membrane; c./. caudal tin; </./. dorsal finlets ; jug.pl. 

 jugular plates : ,//. nostril; i><'t..f. pectoral fin; j><: /. 

 pelvie fin ; v. /. ventral fin. (After ( 'uvier.) 



-" *{f 



B 



Juff.pl 



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

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



