180 THE VERTEBRATE SKELETON. 



7. the frequent occurrence of a pre-dentary or mento- 

 meckelian ossification in the mandible. 



The skull of Pipa is abnormal, being greatly flattened and 

 containing little cartilage. The fronto-parietals are fused, and 

 there is no sphenethmoid. The quadrates are well developed 

 and the squamosals and parasphenoid differ much from those 

 of other Anura. 



HYOID AND BRANCHIAL ARCHES. 



In larval Amphibia the hyoid and four branchial arches 

 are generally present, and in adult Ichthyoidea they are 

 frequently almost as well represented as in the larva, and 

 are of use in strengthening the swallowing apparatus. They 

 are very well seen in Siredon, and consist of a hyoid attached 

 by ligaments to the suspensorium, followed by four branchial 

 arches of which the first and second are united by a copula 

 (fig. 29, D, 8), while the third and fourth are not. The hyoid 

 is not always the largest and best preserved of the arches, 

 for sometimes as in Spelerpes one of the branchials is far 

 larger than the hyoid. Four branchial arches occur in Siren 

 as in Siredon, but in Proteus there are only three. 



In some larval Labyrinthodontia (Branckiosaurus) four 

 branchial arches are known to occur, and their arrangement 

 is almost precisely similar to that in Siredon. 



In Gymnophiona the remains of only three branchial 

 arches occur in addition to the hyoid. The four arches are 

 all very similar to one another, each consists of a curved rod 

 of uniform diameter throughout. The hyoid is united with 

 the first branchial arch, but has no attachment to the 

 cranium. 



In larval Anura (fig. 29, C) the arrangement of the 

 hyoid and branchial arches is much as in Urodela. In the 

 adult, however, the ventral parts of all the arches unite, form- 

 ing a compact structure, the basilingual plate (fig. 29, B, 1). 



