302 



ZOOLOGY 



Fig. 950.— Skull of Frotriton, one of the 



smaller Stegocephala, magnified. Br. 

 br-anchial^arches ; F. frontal ; Fp, parietal 

 foramen ; iv/. maxilla ; N, nasal ; .^a. nos- 

 tril ; Oc. sclerotic plates ; P. parietal ; 

 Pf. prefrontal ; Pvix. premaxilla ; Socc, 

 supraoccipital. (From Wiedersheim, after 

 Frltsch.) 



very large and form an extremely complete and substantial 



structure, especially remarkable for the way in which the small 



orbit (0) is completely surrounded by bones. In the Stegocephala 



(Fig. 950) the skull is broad and flattened, the supraoccipital 



(s. occ.) double, and the parietals 



(P) and frontals (F) are separate. jP/Kcjfig 



Between the parietals is an tsJuuL Jf 



aperture, the parietal foramen .i..»ffiaSiMi. ' 



(Fp), which probably lodged a 



pineal eye. The eyes were 



sometimes surrounded by a ring 



of bony sclerotic plates (Oc). 



Gill-arches have been found in 



many species. 



The shoulder'- girdle of Urodela 

 (Fig. 951) is chiefly remarkable 

 for the great size of the unos- 

 sified coracoids (A. Co., B. f.) 

 which overlap one another ou 

 the ventral body-wall. The pro- 

 coracoid (Gl) is also large, and 

 there is no clavicle. The 

 sternum (St) is usually a more or 

 less rhomboid plate of cartilage 



between the posterior ends of the coracoids, and there is no omo- 

 sternum. In Necturus, however, the sternum presents a very 

 interesting structure : it is a narrow, irregular, median bar,- 

 sending off branches right and left into the myocommas, a condition 

 of things which suggests its origin by the fusion of abdominal ribs, 

 or supporting structures developed between the ventral portions of 

 the myomeres, just as the true ribs are formed between their 

 dorsal portions. In the Anura the epicoracoids either simply 

 meet one another in the middle ventral line, as in Rana, or 

 overlap, as in the Fire-toad (Bomhinator) and the Tree-frogs 

 {Hyla). The overlapping of the coracoids, in Anura as in Urodela, 

 is sometimes correlated with the absence of an omosternum. In 

 the Stegocephala there is a median ventral investing bone, the 

 inte7'-chwicle, which is connected on each side with the clavicle, 

 and extends backwards ventral to the sternum. There is also, 

 on each side, a bone called the dcithrum, connected with 

 the corresponding clavicle : there is some reason for thinking this 

 to be homologous with the bone usually called clavicle in 

 Teleostomi. 



In the pelvic girdle of the Urodela the combined pubic and 

 ischiatic regions (Fig. 952, P, Is) of the right and left sides are 

 united to form an elongated cartilaginous plate which gives off on 

 each side, above the acetabulum {G), a slender vertical rod, the 



