: THE FROG a7, 
The sphenethmoid or girdle-bone is a bony tube encircling the 
anterior end of the cranial cavity. Ina surface view of the entire 
cranium this bone is partly concealed by membrane bones. 
Ventral and lateral to the prodtic is a large depression, the 
tympanic cavity (the middle ear of higher vertebrates), which in 
the fresh skull is covered laterally by the tympanic membrane or 
drum membrane of the ear. At the bottom of the tympanic cavity 
is an opening, the fenestra ovalis, which leads inside the auditory 
capsule; in the fresh skull the fenestra ovalis is closed by a minute 
disc of cartilage called the operculum, which in turn is joined 
with the tympanic membrane by a small bony and cartilaginous 
rod called the columella. Like the ossicles of the mammalian ear, 
these two small structures serve to convey sound waves from the 
tympanic membrane to the inner ear. In prepared skulls the 
operculum and columella are often missing. 
(2) Membrane bones of the cranium. 
The fronto-parietals are two long flat bones meeting in a 
median suture and forming the roof of the cranium. With what 
bones do they come in contact ? 
The parasphenoid or parabasal is shaped like a dagger lacking 
the handle; it lies on the ventral side of the cranium. 
The nasals are two triangular bones on the dorsal surface in 
front of the sphenethmoid; they are set transversely and a little 
obliquely, and form the roof of the nasal capsules. 
The vomers are two bones on the ventral surface of the 
cranium in front of the sphenethmoid. Each bears a group of 
teeth, the vomerine teeth. 
(3) The cartilaginous cranium. Jn a wet preparation from 
which the membrane bones have been stripped off, the unseg- 
mented cartilaginous cranium is exposed to view. On its dorsal 
surface observe the large anterior and two small posterior fonta- 
nelles—window-like openings in the cartilage, closed by mem- 
brane only. Observe that the exoccipitals, prootics and spheneth- 
moid are ossifications in this cartilage. Observe that cartilage 
also forms the basis of both the olfactory and auditory capsules. 
Compare the cartilaginous cranium of the frog with the entire 
cranium of an elasmobranch (e. g., a dog-fish or a skate). 
(b) Jaws and suspensorium. The jaws consist of two carti- 
