408 HARRIS HAWTHORNE WILDER ON 
and median, and lodge, respectively, the horizontal and posterior vertical semi-circular 
canals of the membranous labyrinth. The ampullae of these two canals lie at their 
posterior union and are situated in the deep recess. The large central depression forms a 
portion of the median chamber which contains the membranous vestibule with its large 
otolith. The opisthotic comes in contact with two bones, the paraquadrate and the exoc- 
cipital. The former merely overlaps with its posterior portion the outer margin of the 
opisthotic, but the exoccipital forms, at least im large specimens, a definite union through 
the medium of distinct processes projecting from the two bones. 
The side to which a given opisthotic bone belongs is best determined by the canals 
and the deepest recess. The canals lie nearest to the dorsal surface and the recess is on 
the internal side. 
3. EXOCCIPITAL. This pair of bones, the bodies of which form the occipital condyles, 
embraces and defines the foramen magnum and, with the addition of dorsal and ventral 
eartilagmous arches, entirely encloses it. Each bone consists of a body and two flat 
processes, the supra- and basi-occipital alae. The body is practically identical with the 
condyle, and its posterior aspect forms 
erie dae a rounded surface for articulation with the 
A ala. 
ola atlas. The dorsal or supra-occipital ala 
is nearly perpendicular to the rest of the 
bone and to the floor of the skull and is 
Basi-occipital 
QLa. @ 
set obliquely, at an angleo about 45° with 
both longitudinal and transverse axes of 
the head, so that, when viewed from above, 
POSTE RIOR. veer the median occipital region forms a con- 
on a ote eee Ae EC Ce 3. ; : } ‘ os 
Hig, 10: ‘Two viewsiof the riphtexoceipital 1 spicuous re-entrant angle. This ala arises 
from the body as a narrow stalk which rap- 
idly widens, and thus resembles a triangle resting upon its apex. The dorsal margin, or 
base of the triangle, is grooved for the reception of the cartilaginous arcus supra-occipi- 
talis which spans the median interval between this ala and its opposite. 
The ventral or basi-occipital ala lies exactly in the horizontal plane, coincident with 
that of the parabasale, and is applied so closely to this latter bone by its ventral surface 
that the separation is often attended with some little difficulty. Both W. K, Parker and 
Wiedersheim found the exoccipitals in Proteus actually a part of the parabasal, or, as the 
latter expresses it, ‘‘ synostotisch verbunden.” This is nearly the case in Necturus, but, as 
the two bones are really separable in all cases tried, it is possible that this may be the 
actual condition in Proteus also. The ventral ala is triangular in shape but is attached to 
the body of the bone by a side and not an angle, and, when viewed from the ventral side, 
