I02 ALUS. [Vol. XVIII. 



is deeply hollowed out, throughout its entire length, by the poste- 

 rior portion of the eye-muscle canal, the hind end of which lies at 

 the level of the ventral surface of the basioccipital, and not at the 

 level of the center of the vertebral column. The ventral surface 

 of the basioccipital is cut away in the middle line, throughout its 

 entire length, by a wide rectangular opening, which extends back- 

 ward to the narrow, ventral edge of the solid vertebra-like hind 

 end of the bone, and forms the posterior portion of the hypophysial 

 fenestra {hfn, Fig. 25). The ventral edges of the basioccipital, 

 at each side of the fenestra, are thin, and are overlapped exter- 

 nally by the lateral edges of the hind end of the parasphenoid, the 

 latter bone entirely closing the fenestra. 



The anterior edge of the basioccipital presents three splint-like 

 processes, a dorso-median one and two lateral ones, the dorso- 

 median one being much the stronger of the three. All three proc- 

 esses adjoin, or connect suturally with, the hind ends of the 

 petrosals, the median process overlapping dorsally the adjoining, 

 mesial edges of the mesial, horizontal wings of the petrosals, and 

 the lateral ones each overlapping externally the ventral part of the 

 petrosal of its side. Between the median process and each lateral 

 process, on the dorsal, cerebral surface of the basioccipital, a deep 

 groove begins, and extends from the front edge of the body of the 

 bone backward through about one half its length. The two 

 grooves lie so near the lateral edges of the dorsal surface of the 

 element that only a thin ridge of bone is left, on each side, to form 

 their lateral boundaries. Each groove forms the hind end of the 

 saccular recess {scr, Fig. 24) of its side of the cranial cavity, the 

 anterior end of the recess lying, as already described, on the cere- 

 bral surface of the petrosal. 



Between the hind ends of the saccular grooves, beginning at 

 about the middle of that part of the groove that lies in the basioc- 

 cipital, there is, on the dorsal surface of the latter bone a slight 

 groove. At about the level of the hind ends of the saccular 

 grooves, this median groove becomes a short, median canal, or 

 pit, extending backward and slightly downward into the bone, and 

 ending blindly (Fig. 8). In longitudinal sections cut through 

 this part of the bone a line is seen, which continues the line of the 

 canal backward and downward to the dorsal surface of the conical, 



