SECT. XIII 



PHYLUM CHORDATA 



293 



FIG. 961. Skull of Protriton, one of the 

 smaller Stegocephala, magnified. Br. 

 branchial arches ; F. frontal ; Fp, parietal 

 foramen ; M. maxilla : N. nasal ; Na. nos- 

 tril ; Oc. sclerotic plates ; P. parietal ; 

 Pf. prefrontal ; Pmx. pre maxilla ; Socc. 

 supra-occipital. (From Wiedersheim, after 

 Fritsch.) 



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. 961) the skull is broad and flattened, the supra-occipital 

 (s. occ.) double, and the parietals (P) and f rentals (F) are separate. 

 Between the parietals is an aper- 

 ture, the parietal foramen (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. 962) is chiefly remarkable 

 for the great size of the unossified 

 coracoids (A, Co., B, C.) which 

 overlap one another on the ven- 

 tral body-wall. The procoracoid 

 (Cl) is also large, and there is no 

 clavicle. The sternum (St) is 

 usually a more or less rhomboid 

 plate of cartilage between the pos- 

 terior ends of the coracoids, and 

 there is no omosternum. In Nec- 

 turus, 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 (Bombinator) 

 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 inter-clavicle, 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 cleithrum, 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. 963, 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 

 ilium (II). Ossifications are formed in the iliac and ischiatic 

 regions, but the pubic region remains cartilaginous. The resem- 



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