158 THE ANATOMY OF YERTEBRATED ANIMALS. 



son with the size of the body. In the genera Archegosaurns^ 

 JS^eraterpeton^ Urocordylus, Lepterpeton^ each foot possessed 

 five digits, and the carpus and tarsus were unossified. 



The Amphibia usually possess teeth on the vomers, pre- 

 maxillas, maxillae, and dentary pieces of the mandible, but 

 rarely on the palatine and pterygoid bones. The premaxillary 

 and vomerine teeth are disposed in concentric semicircles, an 

 arrangement which is very characteristic of the group. In 

 the larvae of the J^atrachia^ and in Siren^ the iDremaxillse and 

 mandibles are ensheathed in horny beaks, as in the Chelonia 

 and Aves. In addition, Siren has teeth in the vomers, and 

 on the splenial piece of the mandible ; Menohranchus and 

 8iredon have pterygoid teeth. Many of the Labj^rinthodonts 

 possess palatine teeth. In some Gyynnophiona the mandible 

 has a double row of teeth, and there is an approximation to 

 this structure in the Labyrinthodonts. 



The teeth usually become anchylosed with the adjacent 

 bones. In existing AmpKibia their structure is simple, but 

 in the Labyrinthodonts, the parietes of the teeth, at a certain 

 distance below the summit, become longitudinally folded, and 

 each fold may be again longitudinally plaited, so that the 

 transverse section of the tooth acquires a very complicated 

 structure, the pulp-cavity being subdivided into a great many 

 radiating and branching segments. The structure is similar 

 in principle to that exhibited by the teeth of many of the 

 Ganoidei. In many of the LabjTinthodonts, again, two of 

 the anterior mandibular teeth take on the form of lono;- tusks, 

 which are received into fossee, or foramina, of the upper jaw, 

 as in most existing Grocodilia. The tongue is fixed to the 

 floor of the mouth in TIrodela and GymnopTiiona^ and remains 

 undeveloped in the genera Pipa and Dactylethra^ which have 

 thence been termed Aglossa. In other J^atrachia, the tongue, 

 which is usually long, and fixed by its anterior end to the sym- 

 physis of the mandible, can be rapidly protruded and used as 

 an organ of prehension. No distinct salivary glands have 

 been observed in the Amphibia. Many male JSatrachia have 

 the mucous membrane of the floor of the mouth produced into 

 pouches which can be distended with air. 



The simple alimentary canal is usually short, and much 

 longer in the larvae (which are vegetable-feeders) than in the 

 adults. A gall-bladder is always present. 



The heart presents two auricles, a single ventricle and a 



