14 ILLINOIS BIOLOGICAL MONOGRAPHS [184 



and the posterior part of the floor of the brain cavity. On its dorsal 

 surface (floor of the brain case) are two longitudinal grooves, one on 

 each side of the median line, lying along the lateral walls of the bone 

 and separated by a rounded ridge in the median line. These grooves 

 are open tliroughout the anterior half but are covered in the posterior 

 half by the edges of the exoccipitals which overlap into the brain cavity 

 and with tlie groove form recesses on each side as it passes backwards 

 on each side of the pit of the cavum sinus imparis, reaching almost to 

 the conical, vertebra-like, depression on the posterior end. The cavum 

 sinus imparis is a deep pit on the dorsal surface in the median line near 

 the posterior end, only separated by a single thickness of bone from the 

 conical, vertebra-like, depression. This pit lies on the surface of the 

 median rounded ridge between the lateral grooves which form the re- 

 cesses in connection with the overlapping mesial edges of the exoccipi- 

 tals. As the cavum sinus imparis passes backward it gradually becomes 

 deeper until it forms a saccular cavity at the posterior end. The ventral 

 surface of the bone possesses a long groove the ventro-lateral edges of 

 which almost form a circle in connection with the parasphenoid. This 

 groove tapers gradually into a narrow tube which passes to the posterior 

 end and opens to the exterior by a slit-like opening on the ventral sur- 

 face. This tube forms the posterior portion of the myodome. On the 

 lateral surface of the bone is a rounded ridge, arising at the dorso- 

 anterior edge, where it is a continuation of a similar ridge from the 

 prootic, and extending postero-ventrad nearly to the ventral surface of 

 the bone. The posterior end is rounded, with a slight conical depression 

 which forms the facet for the articulation with the first vertebra. 



Myodome. — The myodome is a long funnel shaped cavity, formed 

 by the basisphenoid, the two prootics, the parasphenoid, and the basi- 

 occipital. The funnel is triangular in outline, located between the two 

 prootics and ventral to the basisphenoid, the pedicle of which bisects 

 the anterior end of the cavity. It becomes narrow as it extends caudad 

 into the basioccipital where it tapers to a tube and opens to the exterior 

 by a slit on the ventral surface. There is an oval opening between the 

 basisphenoid and the partition of the prootics for the pituitary body. 

 Immediately posterior to this opening on the dorso-lateral surface of 

 the myodome there is a pocket which extends upward toward the 

 trigemino-faeial chamber and is only separated from it by a thin parti- 

 tion. Here the myodome is widest and narrows rapidly caudad until it 

 forms the tube. At the median ventral edges of prootics and basi- 

 occipital is a long slit-like opening, the hypophysial fenestra, covered by 

 the parasphenoid, except at the posterior end where it opens to the 

 exterior. The myodome contains the eye muscles, which are attached 

 to the walls and roof of the cavity and pass cephalad into the orbit. 



