ART. 26 REMAINS OF FOSSIL PORPOISES KELLOGG 17 



vaiying in outline and in extent in the different skulls. The ali- 

 sphenoid is overridden anteriorly by the internal plate of the ptery- 

 goid, thus closing the sphenoidal fissure laterally. The optic canal 

 while confluent with the sphenoidal fissure, nevertheless has its course 

 marked by a definite groove, and is bounded anteriorly by the de- 

 scending portion of the orbitosphenoid. No trace of a posterior par- 

 tition or taenia metoptica could be found in any of these skulls. As 

 in Delphinccpterus the foramen rotundum appears to be situated in 

 the angle formed by the anterior margin of the alisphenoid where it 

 comes in contact with the frontal. In some skulls of Delphiimpterus 

 there is a well-defined anteriorly directed canal leading from this 

 angle for conveying the maxillary branch of the trigeminal nerve. 

 This canal is distinct from a wider channel leading from the sphe- 

 noidal fissure, but terminates at the posterior margin of the broad 

 trough fof the optic nerve on the ventral face of the supraorbital 

 process of the frontal, A similar canal or groove can be made out 

 on one or two of these fossil skulls, but the interval between this 

 canal and that leading from the spenoidal fissure is much reduced. 

 When the pterygoid is in position the foramen rotundum as well as 

 its canal and the sphenoidal fissure are hidden from view. Near the 

 base of the alisphenoid and partially overlapped by the vaginal 

 process of the pterygoid is the ectal orifice of the canal for the 

 carotid artery. The mandibular branch of the trigeminal nerve 

 issues through a cleft on the posterior margin of the alisphenoid at 

 a point 9 mm. external to the carotid canal and on its outward 

 course occupies a channel on the ventral face of this bone, finally 

 emerging in the temporal fossa through the foramen ovale. The 

 latter is situated between the falciform process of the squamosal and 

 the parietal, immediately behind the posterior extremity of the ptery- 

 goid. In front of the carotid canal and the channel for the mandi- 

 bular branch of the trigeminal nerve, the alisphenoid curves abruptly 

 upward forming with internal plate of the pterygoid a large con- 

 cavity. This concavity may be further divided into a semicircular 

 internal portion and an elongate external portion. The posterior 

 margin of the internal plate of the pterygoid extends obliquely across 

 this concavity. 



In this region the wall of the cranium consists of three layers of 

 bone. These, from the inside outwards, are: First, the alisphenoid. 

 which occupies the interval between the frontal, parietal, and basi- 

 sphenoid; next, the pterygoid proper, that is, the internal plate which 

 overspreads the sphenoidal fissure, overlaps the lateral margin of 

 the basisphenoid, and contributes the lower outer wall of the nasal 

 passage; and, lastly, the external reduplication of the pterygoid 



9119—25 3 



