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 
ttmea 
Fic. 87.—Chondrocranium of a frog after metamorphosis, from Gaupp. ov, fenestra 
ovalis; m, Meckel’s cartilage; mg, metapterygoid; nc, nasal capsule; pigq, pterygoquadrate; 
tnas, tectum nasalis; tsyn, tectum synoticum; ttmed, tenia 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 
Fic. 88.—Dorsal and ee views of male toad, Bufo a americanus. For letters see 
fig 
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 
