590 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



Four foramina are situated on the posterior part of the walls of the 

 cranial cavity. These probably represent the fenestra ovalis, the foramen 

 lacerum poster ius, and the foramen carotideum. The first named is quite 

 large, and perforates the posterior part of the exoccipital bone, a part of 

 its posterior border being formed by the crested sphenoid, and a small 

 part of its interior lower margin being probably contributed by the 

 basisphenoid. An inferior tongue like prolongation of the exoccipital 

 bone separates it from a large foramen in front of it, which it bounds 

 in conjunction with the presphenoid and prootic. This foramen is oval, 

 with the long axis directed upward. Between it and the fenestra oralis, 

 the exocciptal is pierced by a much smaller round foramen, at a point 

 below the middle of the former. The prootic bone is prolonged for- 

 ward, and at a point much anterior to the exoccipitals, and the remaining 

 part of the supraoccipitails bounds another foramen of not large size. 

 This perhaps gave exit to one of the hrauches of the trigeminus. The 

 anterior extremity of this part of the skull is very peculiar. The sphe- 

 noid, from an ascending direction, turns horizontally, while the supraoc- 

 cipital rises apparently as a median ascending process free from the 

 inferior walls. The latter acquires another roof, which incloses an 

 open cavity with the supraoccipital, which expands forward, and has 

 its lateral borders composed of the united produced lateral angles of the 

 inferior and superior bounding surfaces. The brain-chamber turns for- 

 ward, and the superior part of it terminates rather abruptly. Tbe infe- 

 rior part of the cast of the matrix, which occupies it, is continued with a 

 subtriangular section, resembling the hypophysis, or the united peduncles 

 of the olfactory lobes. The roof of this chamber rests on an osseous 

 mass in front, which is concave above from side to side ; below, its broken 

 section is transverse, its vertical diameter small, and least in the middle. 



There is some uncertainty attending the determination of the ele- 

 ments which compose the mass above described. It is possible, as 

 already observed, that the recurved, cup-shaped basal bone is the sphe- 

 noid, and not the basioccipital. This interpretation receives some coun- 

 tenance from those offered in explanation of the few crania of Dinosau- 

 via hitherto found. These are two, or perhaps three, viz, one described 

 by Mr. Hulke, and a second by Prof. H. G. Seeley in the Quarterly 

 Journal of the Geological Society of London ; and another published 

 near the same time by Dr. Buuzel in the quarto of the K. K. Mineral- 

 ogische Anstalt of Vienna. 



In all of these, the basicranial axis is deflected immediately in front 

 of the occipital condyle, in Dr. Seeley's specimen to a very great ex- 

 tent, as much as in the Crocodilia of later periods. In the other two 

 crania, the deflection is less marked, and it terminates in an angle, from 

 which the axis continues forward. In Mr. Hulke's specimen, it rises 

 somewhat as in the Montana animal. In none of the crania is the ele- 

 ment in front of the condyle recurved as in the latter, though they dis- 

 play in their angle a rudiment of the prominent crest above described. 



