94 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



side. These fuse, forming a ring ('os en ceinture') around the olfactory 

 nerves and the anterior end of the brain. 



The frontals and parietals of a side are fused and often the fronto-parietals 

 are continuous across the middle line. They may extend to the nasals or there 

 may be a gap between, leaving the sphenethmoid visible from above (fig. 97). 

 A large squamosal extends above the quadrate, from the otic region to the angle 



t trried 



rtas 



Fig. 96. — Chondrocranium of a frog after metamorphosis, from Gaupp. Uv, fenestra 

 vestibuli; m, Meckel's cartilage; mig^ metapterygoid; nc, nasal capsule; ^igq, 

 pterygoquadrate; inaSj tectum nasalis; isyn, tectum synoticum; timed, taenia tecti 

 medialis; III-V, nerve exits. 



of the jaw. The upper jaw consists of premaxillary and maxillary, and, except 

 in the aglossa, of quadratojugal. The pterygoid cartilage persists, but is over- 

 laid by a membrane bone, also called the pterygoid. Slender palatines, trans- 

 verse to the axis of the skull, are lacking only in the aglossa, while small vomers 

 are almost always present. The floor of the cranium is completed by a 1-shaped 

 parasphenoid, which extends to the premaxillaries in the aglossa, elsewhere 

 only to the sphenethmoid. 



Fig. 97.— Dorsal and ventral views of skull of toad, Bufo americanus. d, dentary; 

 eo, exoccipital; fp, fronto-parietal; mm, mento-Meckelian; mx, maxillary; w, nasal; pi, 

 palatine; pm, premaxillary; prot, prootic; /)5,;parasphenoid; pt, pterygoid; qj, quad- 

 ratojugal; se, sphenethmoid; sq, squamosjil; », vomer 



In the lower jaw there is a mento-Meckelian in front, followed by dentary 

 and angulare; Meckel's cartilage persists through life. The larval branchial 

 and hyoid arches are typical, there being four gill arches. With the loss of gills 

 the posterior arches disappear, and the broad hyoid plate of the adult has four 

 processes which are new formations. 



