AND CLASSIFICATION OF THE FOSSIL REPTILIA. 
220 
versely, with the fracture passing through the internal carotid canal (fig. 2). This canal 
opens on the cerebral surface by a circular foramen, descends vertically, and forks in 
an inverted Y-shape, so as to have lateral external openings in front of the middle of 
the inferior descending hypapophyses. So much of the base of the brain-case as is 
seen is smooth and flat. It is contracted at the anterior corners by lateral tubercles, 
which may mark the Ihnit of the cerebellum. Behind them the bones enclose a sub- 
hemispherical cavity, which is 1 '9 centim. wide and about as high, though the height 
is probably reduced by crushing. The anterior border of this cerebral chamber is 
smooth, and indicates a vertical lateral vacuity in the skull. The ex-occipital bones 
appear to extend upward and forward in front of the vacuity, forming a concave root 
to the back of the brain, but giving off on each side a lateral process which extends 
forward beneath the so-called supra-occipital bone. This bone (tig. 1) is small on the 
occipital surface, is narrow, divided by a vertical suture, and situate high above the 
foramen magnum, has only a linear longitudinal exposure on the median line of the 
roof of the skull (fig. 2), and extends forward beneath the spatulate bones, which have 
commonly been regarded as the parietals. The height of the brain-case where the 
ex-occipital bones terminate in front is 2‘7 centims. And then the (?) supra-occipital 
bone comes into the roof of the cerebral chamber anteriorly, without increasing its 
width, so that the transverse measurement of the bones, with the median longi¬ 
tudinal interspace which divides them, remains 1'6 centim. The bone forms two 
distinct parallel plates, which are subtriangular, compressed laterally, and at the 
anterior fracture are 1’7 centim. high, with the internal surfaces vertical and parallel, 
and divided by a space 7 millims. wade, which is narrower at the superior border, 
where a strip of the supra-occipital, 2 millims. wide anteriorly, and 5 millims. wade 
posteriorly, is exposed. The width of the bones posteriorly at the summit of the 
occipital plate is about 1'5 centim., increasing suddenly below the median posterior 
groove to 2’5 centims. Their height above the occipital condyle, posteriorly, exceeds 
3 centims. It is impossible not to recall the deso’iption of Loxomma given by Dr. 
Embleton and Mr Atthey* in relation to these bones. They remark, “ The upper 
border of the occipital -surface is also the posterior border of the middle part of the 
skull.” “ It is formed externally by the mastoids, and between them by a pair of bones 
corresponding to those which, in Ai'chegosaurus, are called by von Meyer . . . supra- 
occipitals. Immediately below this border runs a transverse line of suture connecting 
the bones forming the border with those beneath it, namely, next the median line with 
the single, and, as we deem it, the true supra-occipital, and laterally with the ex- 
occipitals. The supra-occipital is of a subtriangular form, wider from side to side than 
from above downwards, and situated on the median line. It is doubtful whether or 
not the median suture passes through it. Below, it articulates with the ex-occipitals.” 
When the external surface of the specimen Pt. 1021 is compared with the corre¬ 
sponding portion of B. 868 there is a close resemblance, though the latter shows the 
* ‘ Ann. ]\Iag. Nat. Hist.,’ July, 1874, p. 50. 
