179] OSTEOLOGY OF LORICATI—GUTBERLET 9 



posterior part of the floor of the brain ease, and extends dorso-cephalad 

 until it reaches the processes which support the T-shaped basisphenoid, 

 leaving a small wedge-shaped slit between the basisphenoid and the 

 partition for the opening of the pituitary body from the brain case. 

 Near the lateral wall, just posterior to the process which supports the 

 basisphenoid, a small foramen for the sixth nerve preforates the parti- 

 tion. Between the prootic, alisphenoid, and the dorso-lateral process 

 of the parasphenoid there is a large foramen through which the anterior 

 cerebral vein, the ciliary nerve, and ciliary artery pass. On the front 

 edge of the foramen and slightly mesad, at the base of the process sup- 

 porting the basisphenoid, there is a second small foramen for the third 

 nerve. On the lateral surface of the prootic is the prootic arcade, which 

 is located slightly dorso-posterior to the foramen between the bones just 

 described and directly ventral to the anterior articular facet for the 

 hyomandibular on the sphenotic. This arcade partly shelters two fora- 

 mina. The anterior and larger one is for the passage of the jugular 

 vein, the external carotid artery, the fifth and part of the seventh nerves. 

 The posterior one is for the hyomandibular part of the seventh nerve. 

 There is another small foramen for the fourth nerve just anterior to the 

 arcade. The arcade forms an arch through which the jugular vein and 

 external carotid artery pass after they have passed through the fora- 

 men. On the inner surface the brain case is smooth with a large broad 

 ridge, produced by the broad groove on the external surface, extending 

 from near the middle just behind the arcade to the posterior end of the 

 bone (where it unites by suture with the exoccipital and the ventral 

 most edge of the pterotic) and continuing caudad as the ridge on the 

 exoccipital. Directly ventral to the ridge on the internal surface, there 

 is a deep depression or groove parallel to the ridge and extending the 

 length of it bordering on the floor of the brain case. This groove con- 

 tinues caudad into the basioccipital where it finally forms a recess with 

 the overhanging edge of the exoccipital. Directly beneath the arcade 

 there is a flange-like process extending from the floor of the brain case 

 to the dorsal edge of the bone where it unites with a similar process of 

 the sphenotic. This flange-like process forms two recesses for parts of 

 the anterior semicircular canal. 



Alisphenoid. — The alisphenoids (Fig. 6, als) are small, irregu- 

 lar bones, ventral to the articulation of the sphenotic and frontal where 

 they are held in place by the clasp of the ventral flange and slender 

 ventro-mesal process of the frontal. Each also comes in contact, ventrad 

 and caudad, with the parasphenoid and prootic. The outer surface is 

 porous while the inner surface is smooth and forms the latero-anterior 

 part of the brain case. A large foramen, postero-ventral to the alisphe- 

 noid, and between it and the prootic and the dorso-lateral process of 



