AND CLASSIFICATION OF THE FOSSIL REPTILIA. 
227 
That bone forms the npper two-thirds of the foramen magnum, and its height above 
the foramen is 3 centims. At the middle of the superior border there is a median 
depression of the usual V-shape, but shallow and wide. On the anterior aspect (fig. 2) 
there are sutural lines somewhat undulating but nearly horizontal, and about 2 cen¬ 
tims. below the superior margin of the plate, showing that the ex-occipital bones are 
overlapped externally by the supra-occipital; and this circumstance may account for 
the larger dimensions of the ex-occipital bones in the external surfaces figured by 
Sir R. Owen. 
A transverse suture appears to extend outward from the hyp-apophysis over the 
lateral notch external to it, separating an anterior plate of bone wliich rests upon the 
ex-occipital, and would meet the basi-occipital internally, forming the posterior w^all of 
a canal which descends oblicpiely outward and downward from the sphenoidal region. 
This ossification enters into the transverse ex-occipital process termed by Owen par- 
occipital, but I cannot trace it superiorly. In Dicynodon lacertice'ps the left hyp-apo¬ 
physis shows a tripai’tite structure, and the outer anterior element I regard as the same 
a,s the imperfectly indicated ossification just described. It is obviously in the position of 
the opisth-otic bone. It must meet the basi-sphenoid if it is not an extension of that 
bone ; and its relations posteriorly with the ex-occipital, and internally with the basi- 
occipital, its combining with those bones in several other specimens to form an articular 
cup for the malleus, which bone extemls transversely outward to the quadrate, favour 
its identification as an otic bone. It is possible that the thickened smooth convex 
surface which extends upward in this specimen from the sphenoidal region on each 
side, anteriorly to the foramen magnum, may be the squamous extension of the pro- 
otic bone. The body of the sphenoid is broken away, leaving a rough triangular 
surface with concave sides, which is less than 1 centim. wide superiorly, 3'3 centims. 
■wide at the base, and 3 centims. in vertical depth. 
Above this fracture the anterior aspect of the specimen shows a portion of the 
posterior walls of the brain-case, which has a high subtriangular outline. The 
foramen magnum is filled with matrix. In front of it the cerebral cavity expands 
vertically to a height of 3 centims. ; and the transverse width increases, towards the 
base of the foramen, to about 2 centims. ; but its lateral border is undefined in the 
middle for 1 A, centim., because the internal wall of the cerebral chamber rounds 
convexly into the lateral external surface of the ex-occipital boundary of the temporal 
vacuity in a way which indicates the absence of bone and the existence of a vertical 
vacuity in the cranial wall, which I regard as that of the fifth and optic nerves. The 
lateral walls of the cerebral chamber here exposed are flattened and oblicjue, so as to 
converge backward toward the foramen magnum, and upward. Superiorly the sides 
round into a slightly flattened convex surface, which is inclined downward and 
backward to the foramen magnum. At the base, the brain-case appears to sink into 
a depression in the line of the basi-occipital bone, but this may be the result of 
development with the chisel. Superiorly two diverging processes, like the forks of an 
2 G 2 
