No. I.] THE SKULL IN' THE MOSASAURID.E. \\ 



The skull as a whole is of the pattern of the Varanidae, and 

 there is no other group of reptiles to which it shows greater 

 resemblance. Seen from above, we have the same form, the 

 same foramina ; the only difference is, that the orbit is com- 

 pletely closed behind by the jugal. Seen from the side, we 

 have the same arrangement as in Varanus, but again the orbit 

 is closed behind. The palatal aspect is also the same ; but the 

 pterygoids come nearer together, and the internal process of 

 the pterygoids is more developed. The principal difference 

 consists in the pterygoid, which bears teeth in the Mosasau- 

 ridae, but none in Varanus. 



TJie Basioccipital (Fig. 1 1). 



The basioccipital is a short element, suturally united in 

 front to the basisphenoid. The occipital condyle projects very 

 little behind, resembling Varanus in this way. It is principally 

 formed by the basioccipital, the exoccipitals forming the outer 

 portion. This element offers only a small base for the brain, 

 the exoccipitals being approached. The canalis basioccipitalis 

 medianus may be present or absent ; the ventral portion is 

 absent in one specimen, there being only a fossa basioccipitalis 

 mediana. The basioccipital processes are well developed, and 

 partially covered by the posterior processes of the parasphenoid. 

 The basioccipital is connected with the basisphenoid, the ex- 

 occipital, and the posterior base of the petrosal. 



The Basisphenoid (Fig. 1 1). 



The basisphenoid is more complex. It is co-ossified with the 

 parasphenoid, which forms the anterior slender process, and 

 which covers the basioccipital processes. The pterygoid proc- 

 esses are short, but broad horizontally. On each side of the 

 basisphenoid is a deep groove. These grooves unite in front 

 into a large transverse foramen below the pituitary body. This 

 foramen may pierce the basisphenoid in the region of the pit- 

 uitary fossa ; a vertical canal may also be present between the 



seen, sticking out of a chalk bank. Gradually I developed nearly the complete 

 animal. I take the opportunity here to thank Mr. David Bower, formerly of Russell 

 Springs, Kansas, for much assistance he gave me during my stay at his house. 



