86 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



with the columella, which meets the tympanic membrane. This membrane is 

 stretched on a cartilaginous tympanic annulus, derived from the pterygoquadrate. 

 (Annulus and columella are lacking in those genera, Bombinator, etc., which have no 

 tympanum). There is no connexion between stapes and quadrate. 



The chondrocranium largely persists, the only constant cartilage bones being 

 the exoccipitals and prootics. A supraoccipital rarely occurs and basioccipital and 



nets 



FIG. 87. Chondrocranium of a frog after metamorphosis, from Gaupp. fov, fenestra 

 ovalis; m, Meckel's cartilage; mtg, metapterygoid; nc, nasal capsule; ptgq, pterygoquadrate; 

 tnas, tectum nasalis; tsyn, tectum synoticum; timed, tsenia tecti medialis. 



basisphenoid are unknown. In the ethmoid region, except in the aglossa, there is 

 a peculiar bone, the sphenethmoid, which arises as two bones on either 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 



FIG. 88. Dorsal and ventral views of skull of toad, Bufo americanus. For letters see 



fig. 68. 



be a gap between, leaving the sphenethmoid visible from above. A large squamosal 

 extends above the quadrate, from the otic region to the angle of the jaw. The upper 

 jaw consists of premaxillary and maxillary, and, except in the aglossa, of quadrato- 

 jugal. The pterygoid cartilage persists, but is overlaid by a membrane bone, also 

 called the pterygoid. Slender palatines, transverse to the axis of the skull, are 

 lacking only in the aglossa, while small vomers are almost always present. The 



