2 so CLASSIFICATION OF VERTEBRATES. 



BODIPTERINI, with two dorsals, partially ossified vertebral centra; diphy-or 

 heterocercal tail ; two large and several smaller gular plates. Osteolepis, 

 Devonian, Europe. The Polypterid.-e (Cl.4.distia) with two living 

 genera (Polypterus from the Nile, Calamoichthys, greatly elongate and 

 lacking ventral fins, from Old Calabar) are most closely allied to the last. 

 The vertebras are ossified, the caudal diphycercal ; the dorsal fin elongate 

 and divided into finlets ; pectorals with a short, scaled basal axis ; the body 

 covered with rhomboid ganoid scales. No fulcra exist. In the skull epi- 

 and opisthotics are not distinct; there are two sphenoidals and ecteth- 

 moids ; the parietals and frontals are paired, and the gular plate is double. 



ORDER II. CHONDROSTEI. 



In the sturgeons and paddle-fish there is but slight ossifica- 

 tion of the cartilage, the vertebral centra being unossified, while 

 in the chondrocranium only otic and ectethmoid ossifications 

 appear. The skull is covered with membrane bones, the parie- 

 tals and frontals being paired, while the large parasphenoid 

 extends back beneath the anterior vertebrae. A premaxilla is 

 absent, and only a dentary is present in the lower jaw. The 

 jaw itself is suspended by sympletic and hyomandibular carti- 

 lages, both partially ossified, the mouth itself being ventral as 

 in elasmobranchs. The operculum is large, but its elements are 

 poorly developed ; the branchiostegals are weak or wanting. 

 The body is either naked or covered with rows of bony plates, 

 ■which are continued upon the upper lobe of the heterocercal tail, 

 upon which fulcra are also strongly developed. The ventral 

 fins have a row of cartilaginous basalia. 



Two recent families are recognized. In the AcipenseriDjE or sturgeons 

 the body is covered with five rows of keeled bony plates, the skin between 

 -.the rows bearing small granules. The mouth is toothless in the adult, and 



Fig. 252. Common sturgeon, Acipenser stiirio, after Goode. 



in front of it are four barbels. The gill slits are four in number, and the 

 operculum, which does not completely cover the slits, bears an accessory 

 gill. The air-bladder is large and simple, and the stomach has pyloric 



