SKELETON 



93 



four rostrals form a ring around the suctorial mouth and recall the labial 

 cartilages of the elasmobranchs and the annular cartilage of the cyclostome 

 mouth. 



At the time of metamorphosis the changes are great, and as the result 

 (fig. 96) is more like the chondrocranium of other amphibia, the larval condi- 

 tion must be regarded as adaptive rather than ancestral. The suprarostrals 

 disappear and the jaw shifts the hinge back to the normal position, this being 

 accompanied by the elongation of Meckel's cartilage, an absorption of the 

 ascending process and a folding of the pterygoquadrate bar. At the same time 



Fig. 95. — Chondrocranium of tadpole of Rana before the metamorphosis; after 

 Gaupp. c.,anl, anterior canal; car, foramen for carotid; els, superior labial cartilage; 

 ctr, cornu trabeculae; ext. c, external canal; /e, ethmoid fenestra; m, Meckel's cartilage; 

 pc, posterior canal; po, otic process of quadrate; pros, ascending process of quadrate; 



!', quadrate; tm, tectum medialis; Urn, taenia tecti marginalis (raised border to side of 

 etters); tsym tectum synoticum; I-V, nerves and nerve exits. 



a pter>'goid grows out in front to join an antorbital process from the cranium. 

 A stapes develops and connects with the columella, which meets the tympanic 

 membrane. This membrane is stretched on a cartilaginous tympanic annulus, 

 derived from the pterygoquadrate. (Annulus and columella are lacking in 

 those genera, Bombinaior, etc., which have no tympanum). There is no 

 connexion between stapes and quadrate. 



The chondrocranium largely persists, the only constant cartilage bones being 

 the exoccipitals and prootics. A supraoccipital rarely occurs and basioccipital 

 and basisphenoid are unknown. In the ethmoid region, except in the aglossa, 

 there is a peculiar bone, the sphenethmoid, which arises as two bones on either 



