po ALUS. [Vol. XVIII. 



large and almost completely enclosed circular opening being 

 formed which gives passage to the external semicircular canal. 



The ventral edge of the petrosal is cut out by a semicircular 

 opening, the internal carotid incisure, which lies opposite the 

 angle formed where the hind edge of the lateral wing of the para- 

 sphenoid joins the lateral edge of the body of that bone. Through 

 this opening, the internal carotid artery enters the eye-muscle 

 canal, where it runs forward to the anterior edge of the basisphe- 

 noid, there turning upward to enter the cranial cavity through the 

 orbital opening of the brain case. 



The Parasphenoid (PS) covers the ventral surface of the 

 primordial cranium through nearly its entire length, extending 

 from close to the hind end of the basioccipital bone forward almost 

 to the anterior end of the antorbital cartilage. The bone consists 

 of a long, thin body and two lateral ascending processes or wings, 

 one on either side. The lateral processes lie at about the pos- 

 terior third of the bone. Immediately in front of them the bone 

 is narrow and stout, widening gradually, and becoming gradually 

 flatter and thinner, toward its anterior end. Between and behind 

 the wings the bone is broader, curves gradually upward and back- 

 ward to its hind end, and upward and laterally, on each side, to 

 its lateral edges. This part of the bone is thus strongly convex 

 on its ventral surface and concave on its dorsal surface. In the 

 median line of the posterior half of this posterior portion, on 

 its dorsal surface, there is a deep longitudinal channel the nearly 

 parallel sides of which correspond approximately with the edges 

 of the hypophysial fenestra. It is deepest in its middle portion, 

 gradually disappearing both anteriorly and posteriorly. It forms 

 the bottom of the eye-muscle canal. At the hind end of the 

 parasphenoid the channel occupies nearly the full width of the 

 bone. The hind edge of the bone is thin and usually frayed. 



The lateral wings of the parasphenoid are flat, curved, and 

 sharply pointed processes, directed upward, forward and laterally. 

 The anterior edge of each process is concave, its posterior edge 

 convex. The hind edge overlaps externally the anterior part of 

 the ventral edge of the petrosal, the sharp dorsal point of the 

 process not quite reaching the level of the ventral surface of the 

 horizontal process of the petrosal. The concave anterior edge 



