18*2 THE VERTEBRATE SKELETON. 



cornua, which are united to the periotic region of the skull iii 

 front of the fenestra ovalis either by short ligaments or by 

 fusion as in Bufo. In Pipa and Xenopus the first and 

 second branchial arches persist as well as the fourth (thyro- 

 hyal), but in Pipa the hyoid is wanting. 



KIBS. 



Ribs are generally very poorly developed in Amphibia. 

 In Anura they are in most cases absent; when present they 

 generally form minute unossified appendages attached to the 

 transverse processes, but in Discoglossus and Xenopus the 

 anterior vertebrae are provided with distinct ribs. In 

 Urodela and Labyrinthodontia they are generally short 

 structures, each as a rule attached to the vertebra by a 

 bifurcated proximal end. The number of rib-bearing vertebrae 

 varies, but the first and the posterior caudal vertebrae are 

 always ribless. The anterior caudal vertebrae too are gene- 

 rally ribless, but sometimes a few of them bear small ribs. 

 In Spelerpes the last two trunk vertebrae are ribless, and 

 hence may be regarded as lumbar vertebrae. 



In Gymnophiona ribs are better developed than in any 

 other Amphibia; they occur on all the vertebrae except the 

 first and last few, and are attached to the transverse processes, 

 sometimes by single, sometimes by double heads. 



Sternal ribs are almost unknown in Amphibia, but traces 

 of them occur in Menobranclius. 



STERNUM. 



In Amphibia the sternum is not very well developed; 

 sometimes as in Gymnophiona and Proteus no traces of it 

 occur, and in the Urodela it is never ossified. It is always 

 very intimately related to the pectoral girdle. In the Sala- 

 mandrina it has the form of a broad thin plate of cartilage, 

 grooved and overlapped by the coracoid. 



In most Anura the sternum consists of a number of parts 



