272 PROCEEDINGS OF THE CANADIAN INSTITUTE. 
foramen, small, situated on a line with the inwardly projecting 
ledges, and giving passage to the first spinal nerve. The inner sur- 
face of the bone is smooth. The exoccipitals do not unite into a 
close articulation with neighbouring bones, but are merely placed in 
apposition, the outlines of the bones not being indented but perfectly 
smooth. They are in relation above with the swpraoccipital, epiotics, 
and pterotics ; in front with the prootics ; and below with the bast- 
occipital. 
3. Basroccipitau (Pl. IL., Fig. 2, BO). 
The Basioccipital is shut out by the exoccipitals from contributing 
to the formation of the foramen magnum. Its posterior face is 
deeply concave; below is a nutrient foramen; the upper surface 
forms the floor of the sinus impar; and the body of the bone is 
deeply hollowed for the reception of the sacculus of either side. It 
extends forwards, becoming smaller and thinner anteriorly, where it 
articulates with the posterior edge of the basisphenoid. Its articula- 
tions are as follows:—Above and at the side with the exoccipitals, 
and prootics ; below with the parasphenoid ; in front with the bas?- 
sphenoid ; behind with the body of the first vertebra ; and laterally 
with the horizontal limb of the supraclavicular. 
4, Eprorics (Pl. II., Fig. 1, EpO). 
These bones, one on either side, form the postero-lateral angles of 
the skull. Each has an irregularly spherical triangular shape, affixed 
by the base, the apex forming the projecting angle. Internally the 
bone supports part of the posterior and longitudinal semicircular 
canals, the former passing in a deep groove on its posterior wall, the 
latter lying on the horizontal floor. The anterior upper edge of the 
bone is deeply channelled, the cavity communicating with a similar 
one in the substance of the pterotic. The articulations of the epiotics 
are with the supraoccipital, exoccipital, pterotic, and supraclavicular. 
5. Prerotics (Pl. IT., Figs. 1 and 2, PtO). 
Form the postero-external angles of the skull. Each is an ossifi- 
cation around the arch of the horizontal semicircular canal. The 
posterior upper edge shows a wide opening extending some distance 
into the cavity of the bone, apparently separating the upper portion 
of the bone into two lamellae. The groove mentioned above as. 
occurring on the epiotic, and also one on the outer edge of the hori- 
