1 6 URODELA 



and to the two centralia ; exceptional, for instance, in Crypto - 

 branchus japonicus, are as many as three centralia, but this is an 

 individual, even a one-sided variation, as shown for instance by 

 a specimen in the Cambridge Museum. Loss of the fifth toe 

 occurs sporadically in genera of different groups, namely, in 

 Salamandrella, Batrachyperus, Salamandrina, Necturus, Manculus, 

 Batrachoseps. In Amphiuma the number is reduced to three 

 or two ; in Proteus to two ; and in Siren the hind limbs, 

 with their girdle, are altogether absent. Lastly, in some species 

 of Spelerpes and Batrachoseps both fore and hind limbs have 

 become so small as to be practically without function, parallel 

 cases being found among various Scincidae and other Lizards. 



The hyoid apparatus is still very primitive in many, 

 especially in larval, Urodela. Besides the hyoid there are as many 

 as four pairs of branchial arches, which, however, decrease in size 

 and completeness, so that the last two have lost their connection 

 with the median copular piece, and become attached in various 

 ways to the second branchial arch. This is the arrangement 

 apparently in all larvae, but four pairs of branchials persist in 

 the adult Siren, AmpJiiuma, and Cryptobranchus alleghaniensis. 

 The whole branchial apparatus is reduced to three pairs of 

 arches in Necturus and Proteus, to two in the adult Crypto- 

 branchus japonicus and in the Salamandridae. Of considerable 

 interest is the vestige of a fifth pair of arches in the larvae of 

 Triton and Salamandra, in the shape of a pair of tiny cartilages, 

 which lie in front and on each side of the opening of the 

 trachea, arid give rise to the formation of the laryngeal cartilages, 

 better developed in the higher Vertebrata. 



The following are noteworthy characters of the skull of 

 Urodela. The articulation of the skull with the vertebral column 

 is not always effected entirely by the two condyles of the lateral 

 occipital bones, but the median basal cartilage often possesses a 

 pair of facets for the odontoid-like process of the first vertebra ;. 

 such additional facets are perhaps best developed in Crypto- 

 branchus and in the Salamandrinae. 



The middle portion of the primitive cranium, from the exit 

 of the optic nerve to the ethmoid cartilage, is formed by a pair 

 of separate bones, the orbito- sphenoids. The parietal and 

 frontal bones remain separate. One or more periotic bones 

 exist, besides the prootic, in the aquatic families. 



