4 ON THE STRUCTURE OF TRACHYPTERUS ARCTICUS, 
two lateral processes which enclose a median vertical groove for the reception of the 
parasphenoid. Each process articulates by its anterior edge with the descending 
process of the prootic. Above, the basi-occipital sends upwards a median keel 
which projects between the ex-occipitals and separates those bones, except posteriorly 
in the region of the condyle. The basi-occipital enters into the formation of a pit- 
like hollow in the floor of the brain-case, and here also it is jomed by that median 
cartilage which unites the prootics. 
Fig. 2. Tae Occrpirat anp Otic Bones. 
b.-0c., basi-occipital ; ep., epiotic; ex-oc., ex-occipital ; 
op., prootic; p.sp., parasphenoid; par., parietal; pt., 
pterotic ; s.-oc., supra-occipital; sp., sphenotic; v., VIL, 
foramen for these nerves. 
Epiotics.—The epiotics are very large, the largest of the otic series, and form 
the greater part of the back of the skull. They meet above the foramen magnum 
along a vertical line, an inch long, by a slight cartilaginous intervention which 
expands below to form the dorsal boundary of the foramen magnum. Internally 
the bone presents a regular curved ridge looking upwards, traversing the bone at its 
lower third. This ridge is capped by cartilage, and forms an articulation for the 
supra-occipital and for the frontal, both of which bones, therefore, the epiotic over- 
laps. Below the ridge the bone is thick and deeply pitted, and ends m a thick 
base, the outer portion of which is capped by cartilage, and is received within 
the great wing of the ex-occipital. Externally the epiotic articulates with the 
parietal and the pterotic; anteriorly there arises from the cranial surface of the 
bone a slight process, which uniting with a similar but larger process of the 
prootic, forms a strong pillar on the internal wall of the skull. The epiotic comes 
also into relation, to a slight extent, with the sphenotic. 
The pterotics form the greater part of the ridge between the orbits and the 
posterior wall of the skull. This ridge is sharp and keel-like below, and passes 
up into a broad face above. From the surface the bone passes inwards like a wedge. 
The posterior face of the wedge is covered by the post-temporal, the pit for which it 
enters largely into the formation of. Below and internally this same face presents 
a rough articular surface which receives the outer lower portion of the wing of the 
ex-occipital ; above this the pterotic articulates with the epiotic and the parietal. 
