352 Mr. C. W. Andrews on the Structure 
seen to be closely applied to the ventral surface of the basi- 
sphenoid, and in one young individual the line of separation 
between the two bones is visible. It is a thin plate-like 
structure, and is evidently a membrane bone. 
In consequence of the crushing undergone by all the 
available specimens the arrangement of the bones in the upper 
surface of the skull is very difficult to determine; the present 
description is therefore brief and must be regarded as in part 
provisional. 
The parietals form a high crest between the temporal 
fossee ; they descend considerably on to the side of the brain- 
case, and at the anterior end their lower edge unites with 
the upper end of the epipterygoid. Posteriorly they unite 
ventrally with the supraoccipitals and send out lateral pro- 
cesses, which, together with the upper rami of the squamosals*, 
form the post-temporal bars; but the exact share of the two 
elements in these structures cannot be determined, all trace of 
the suture between them being obliterated. 
The structures here spoken of as lateral processes of the 
parietals have been described by Cope on a skull of Cmolio- 
saurus as distinct elements (supramastoids) ; but both here 
and in the skull of the young Plesiosaur figured in the 
‘Geological Magazine’ for June 1895 there is no trace of any 
suture dividing these processes from the parietals, although in 
the latter specimen the opisthotic is still imperfectly united 
with the exoccipital, and the bones of the basis cranii are still 
separate. 
Between the parietals, immediately opposite the anterior 
end of the temporal fosse, is the pineal foramen, an oval 
aperture of considerable size, looking upward and forward. 
The boundary between the parietals and frontals cannot 
be made out very satisfactorily. It seems to have passed 
just in front of the pineal foramen, part of the anterior border 
of which may indeed have been formed by the frontals: the 
probable position of the suture is shown in woodeut 2A; this 
agrees with the arrangement found in Murenosaurus and very 
nearly with that shown in Owen’s figure of an imperfect 
cranium of Polyptychodon Tt. 
If their line of union with the parietals is rightly deter- 
mined, the frontals have the form shown in woodcut 2 A. 
Their narrow anterior extremity is in sutural union with the 
* The term “squamosal”’is here used in the sense employed by Koken, 
and the triradiate bone designated by it is probably formed by a fusion 
of the supratemporal and squamosal. 
+ “Foss, Rept. Cretaceous Form.,” Suppl. 8 (Mon. Pal. Soc. 1858) 
pl. iv. fig. 1. 
