278 



CLASSIFICATION OF VERTEBRATES. 



Fig. 278. Skeleton 

 of Xecturus. 



The sternum is lacking in the gymno- 

 phiona ; in the urodeles and arciferous 

 anura it is a median plate grooved to 

 receive the epicoracoids in front. In 

 the firmisternous anura the sternum ex- 

 pands in front of the procoracoids and 

 clavicles into an omosternum, behind the 

 coracoids to a xiphisternum which may 

 be partially ossified. 



The skull is noticeable for the great 

 extent to which the chondrocranium 

 persists, and for the wide interval be- 

 tween the trabeculae. This persistence 

 of cartilage accounts for the small num- 

 ber of cartilage bones found in all 

 groups except the gymnophiona. Thus 

 in the anura only a prootic occurs in the 

 auditory region ; in the urodeles an opis- 

 thotic is added. In the occipital region 

 there usually occur but the two exoccip- 

 itals, each bearing an occipital condyle. 

 The quadrate forms the sole suspensor 

 of the jaw, and is more or less closely 

 connected with the otic capsule. If a 

 hyomandibular be present, it is modified 

 into the stapes. In the urodeles no eth- 

 moid ossification occurs, while an orbito- 

 sphenoid is the only bony element in 

 the trabecular region. In the anura a 

 ring-like sphenethmoid occurs (os en 

 ceintiire). In the gymnophiona the eth- 

 moid is very large and has large lateral 

 wings. 



The membrane bones are more nu- 

 merous in the gymnophiona, stegoceph- 

 als, and anura than in the urodeles, the 

 skull being very complete in the first 

 two groups, while in the anura a large 



