394 



Vertebrata. 



skull are retained throughout life^ covered for the most part by 

 membrane bones. There are two articular condyles on the 

 occipital. The premaxillae and maxillae are closely adherent to the 

 anterior solid portion of the skull ; they are not movable as in the 

 Teleosteans. The upper part of the mandibular arch^ the palato- 

 quadrate, is fused to the hinder part of the skull; sometimes^ as 

 in the Anura, it is also fused to the front part by its anterior end ; 

 it remains partly cartilaginous. In the larva there are usually, 

 besides the mandibular and hyoid arches, four pairs of cartilaginous 

 branchial arches, which degenerate to some extent in the 

 metamorphosis; in the Urodela, the first two pairs persist. The 

 basibranchials, tyoid, and branchial arches, are together termed the 

 hyoid. 



Of the skull bones, besides those ah-eady noticed, the following must be 

 mentioned. In the cartilaginous cranium itself there develops a pair of 

 exoccipitals which almost completely surroimd the foramen magnum, and 

 which bear the occipital condyles; anterior to these on either side is the 

 petrosal, and at the front end of the cranium, the sphenethmoid. 

 The skull is covered dorsally by a pair of nasals behind the external nares, and 

 a pair of f rentals and parietals (in the Anura those of each side fuse 

 into a single bone) ; ventrally there is a parasphenoid (c/., Pish) and, anterior 

 to this on each side, the vomer. In the palato-quadrate cartilage 



Fig. 320. Skiill of a Frog {Sana esculenta) . (A) from the dorsal (B) from the 

 ventral surface, c cartilaginous lateral portions of the skull, e sphenethmoid, e' cartilaginous 

 nasal capsule, fn nasal hone, fp fronto-parietal, h' stylo-hyoid, i premaxiUa, j quadrato-jugal, 

 m maxilla, m' quadrate, o exoooipital, op cartilage between the latter and the prootic p, 

 p' anterior portion of prootic, with a large nerve foramen (p"), P^ palatme, pt pterygoid, 

 pi' posterior portion of the pterygoid, s parasphenoid, t — t' squamosal, v vomer. — After 

 Ecker. 



there is, at the point of junction with the lower jaw, an insignificant ossifioati(&i, 

 the quadrate, and behind, the cartilage is covered by a large membrane bone, 

 the squamous; the pterygoid extends anteriorly, and in front of this 

 there is in the Anura a transverse palatine attached to the skull by its inner 

 end. In this group, too, a thin bony rod, the jugal or quadrato-jugal, 

 stretches from the quadrate to the maxilla. The rami of the mandible 

 consist, as in Fish, of several bones. 



