XX] CLASSIFICATION 517 



Order IV. Chondrostei. Osteichthyes in which dorsal and 

 ventral intercalaries are present as well as neural and haemal 

 arches but in which no centra are found, the notochord being per- 

 sistent arid surrounded by an unsegmented cartilaginous tube. 

 Few or no cartilage bones present in the cranium, jaws feeble and 

 toothless connected to the skull only by hyoid. Numerous dermal 

 bones on the head and the pectoral girdle. Clavicles present 

 between the cleithra. Tail fin heterocercal with a row of fulcra on 

 anterior lobe. Conus distinct in heart, optic chiasma in brain. 

 Oviduct short and opening by a wide funnel into the coelom, testis 

 discharging through kidney by vasa efferentia. 'Air-bladder opens 

 into pharynx in the mid- dorsal line. 



Family 1. Polyodontidae. Skin naked or with vestigial scales 

 deeply embedded. 



Ex. Polyodon, Psephurus. 



Family 2. Acipenseridae. Skin covered with five longitudinal 

 rows of bony scutes. 



Ex. Acipenser, ScapMrhynchus. 



Order V. Polypterini. Osteichthyes in which the notochord 

 is surrounded and partly obliterated by amphicoelous bony centra 

 resembling those of Teleostei. Both dorsal and ventral ribs 

 present. Skull well ossified but devoid of supra-occipital bone. A 

 splenial in the lower jaw and a pair of gular plates on the throat. 

 Spiracle persistent and covered with a special cover formed from 

 the postorbital bone. Clavicles present in pectoral girdle, pectoral 

 fin with a median scaly lobe. The skin covered with ganoid scales 

 beset with placoid denticles. A diphycercal tail fin. Conus distinct 

 in heart and optic chiasma in brain. Oviduct short, opening by a 

 wide funnel into coelom. Testis with special duct joining kidney 

 duct near its external aperture. A spiral valve on intestine. Air- 

 bladder bilobed and opening on ventral side of pharynx. 



Ex. Polypterus, Calamoichthys. 



Order VI. Dipnoi. Osteichthyes in which there are intercalary 

 pieces but no centra, the notochord being surrounded by an un- 

 segmented cartilaginous sheath. Few cartilage bones in skull, a 

 median series of dermal roofing bones. The upper jaw fused with 

 cranium, no premaxilla or maxilla. A diphycercal tail, body covered 

 with thin cycloid scales, paired fins typically long and biseriate. 





