XIII 



PHYLUM CHORDATA 



281 



elkj- 



Jbo 



occ.c 



FIG. 894. Skull of Siphon ops annulatus. 



Ang. angular ; occ.c. occipital condyle ; de. 

 dentary ; eth. ethmoid ; /: frontal ; m. maxilla ; 

 na. nasal aperture ; npx. naso-premaxilla ; 

 orb. orbit ; p. parietal ; p.o. ex-occipital and 

 otic bones ; qu. quadrate ; sq. squamosal ; t.d. 

 ducts of tentacle glands. (From Wiedersheim.) 



orbit (Orb.) is completely surrounded by bones. In the Stegocephali 

 (Fig. 895) the skull is broad and flattened, the supra-occipital 

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

 Between the parietals is an 

 aperture, the parietal fora- 

 men (Fp\ which perhaps 

 lodged a pineal eye. The 

 eyes were sometimes sur- 

 rounded by a ring of bony 

 sclerotic plates (Oc.). Gill- 

 arches have been found in 

 many species. 



The shoulder - girdle of 

 Urodela (Fig. 896) is chiefly 

 remarkable for the great 

 size of the unossified cora- 

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

 overlap one another on the 

 ventral body-wall. The pro- 

 coracoid (67) 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 oino- 

 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 sug- 

 gests 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 omoster- 

 num. In the Stegocephali there is a median ventral membrane- 

 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 



-..Skull of Protriton, one of the 



smaller Stegocephali, magnified. Br. 

 branchial arches ; F. frontal ; Ff, parietal 

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

 tril ; Oc, sclerotic plates ; P. parietal ; Pf. 

 pre-f rental ; Puts, pre-maxilla ; Socc. supra- 

 occipital. (From Wiedersheim.) 



