252 



CLASSIFICATION OF VERTEBRATES. 



the alligator gar, L. tristaechus of the southern states, reaches a length of 

 ten feet. Allied fossil forms are numerous, Catopterus being represented 

 in the triassic rocks of the Connecticut valley. Lepidotus ranges from 

 the trias to the Jura of Europe. Aspidorhynchus had a snout something 

 like that of the sword-fish. In the AMIID^E (HALECOMORPHI) the .vertebrae 

 are amphicoelous, the scales cycloid, teeth on pterygoids as well as on 

 vomers and palatines, no pyloric caeca. Amia calva, the bow fin of the 

 eastern U. S., is the only living species. The genus dates from the eocene. 

 Allied fossil forms are Eurycormus, Callopterus, Caturus, and Pachycor- 

 mus, ranging from the lias to the Jurassic. 



Legion II. Teleostei. 



Fishes with the bony skeleton well developed, the cranium 

 and the vertebral centra ossified, the latter amphicoelous ; tail 

 diphy- or homocercal ; spiral valve and conus arteriosus not 

 developed ; no optic chiasma ; scales, 

 when present, cycloid or ctenoid. 



The group of teleosts or bony fishes 

 so closely follows the ganoids that some 

 students do not distinguish between 

 them. There are, however, some dis- 

 tinctions between the two groups, while 

 the matter of convenience warrants 

 their recognition. 



In a few teleosts the skin is naked 

 or covered with bony plates, but usually 

 the body is covered with scales of the 

 cycloid or ctenoid type. In a few the 

 tail is diphycercal, but usually it is hom- 

 ocercal. The fulcra, so characteristic of 

 most ganoids, never occur. The skele- 

 ton is well ossified, this being especially 

 true of the skull, where the cartilages 

 are almost entirely replaced by bone. 

 The operculum and its skeleton are 

 well developed, branchiostegals are pres- 

 ent, and gular plates rarely occur. The 

 paired fins never have a basal lobe ; and the ventrals, when pres- 

 ent, may either be near the vent or far forward, beneath the 



FlG. 254. Breathing 

 valves of teleosts, after 

 Dahlgren. av^ anterior or 

 oral valves open; g; gills; 

 o, oesophagus ; pv, posterior 

 valves. At expiration the 

 anterior valves close, the 

 posterior open ; the enlarge- 

 ment and contraction of the 

 oral cavity being brought 

 about by motion of the oral 

 walls (black). 



