166 MAEINE EEPTILES OF THE OXPOED CLAT. 



tliat of Murwnosaurus, and will be described mainly by pointing out such differences 

 as occur. Unfortunately the material available for description is very imperfect, all 

 the specimens being much crushed and wanting many important parts. 



The hasioccipital {h.oc, PL IX. figs. 1, 2, 4, 5) is closely similar to that oi Muneno- 

 saurus, as will be seen from the figures. The occipital condyle is perhaps a little 

 broader in proportion to its height, and its articular surface is continued quite up to 

 the facet for the exoccipital, there being no trace of a neck to the condyle such as is 

 seen in most specimens of the hasioccipital of Marcenosaurus ; the basipterygoid 

 (lateral) processes {pt.p.) are a little more rounded in section than in that genus. 



The basisphenoid [h.sp., PL IX. figs. 2, 5) does not differ in any important respects 

 from that of Murcenosaurus or Tricleidus. The facet marked/, in text-fig. 73, A, is here 

 very well developed, and probably received the anterior lower angle of the pro-otic. The 

 ventral face is extensively overlapped by the parasphenoid in the usual way. In the 

 young skull (PL IX. fig. 5) the basisphenoid shows evidence of its original ossification 

 from two centres, since it is deeply notched posteriorly in the middle line and there 

 is also a large vacuity beneath the pituitary fossa. This opening corresponds to the 

 original space between the trabeculae, through which the pituitary body was connected 

 with the pharynx ; possibly, even in the young, it was partly closed by the posterior 

 end of the parasphenoid, and it corresponds in position to the pit occurring in some 

 specimens of the basisphenoid of Murcenosattrus (see PL III. fig. 1). 



The structure of the exoccipital-opisthotic {ex.op., PL IX. figs. 1, 1 «, 4, 4 a) differs 



very little from that seen in Murcenosaurus ; the paroccipital process is rather shorter, 



and in the oldest specimen available for comparison (R. 2860) the fossae for the 



ampulla of the posterior vertical semicircular canal and for the canals themselves are 



larger and more open (see PL IX. fig. 1). In the young specimen (R. 2417) the 



exoccipital and opisthotic are united above the jugular foramen [jitg.), theu.' line of 



union being still clearly visible ; but below that opening they are still separate (PL IX. 



fig. 4), each terminating in a distinct facet for union with the hasioccipital. In this 



specimen also the posterior face of each exoccipital bears a small facet (/!) somewhat 



resembling a zygapophysis ; this probably indicates that a well-developed pro-atlas 



v/as present, since the form of the neural arch of the atlas is such that there certainly 



was no point of contact between it and the exoccipitals. The supraoccipital [soc.) is a 



high arch curving forwards, the occipital surface being concave from above downwards 



and continuous with the posterior face of the postero-lateral processes of the parietals 



(PL IX. fig. 1 a). In front the supraoccipital bears a facet looking downwards and 



forwards for union with the pro-otic, and the inner side of this surface is deeply 



channelled for the reception of the upper portion of the posterior vertical semicircular 



canal [p.v.c). Between the pro-otic facet and the junction with the parietal the 



border of the supraoccipital is sharp and concave, and clearly did not unite with any 



other element. 



