FAUNA OF THE G 03 ATT FORMATION". 



633 



longed backward and outward above the otic bones ; but the sur- 

 face is prolonged obliquely outward and downward, so as to form a 

 smooth quadrate area, which rounds towards the base of the skull, 

 and terminates backward on the left side, where best preserved, in 

 a vertical lunate surface, convex behind and concave in front, formed 

 by the exoccipital and otic bones. In front these spaces slightly 

 converge, especially below and above ; and behind the middle there 

 is a foramen a millimetre or two in diameter, apparently largest on 

 the left side, but smaller than might have been expected if it is the 

 entrance to the auditory chamber, which probably lies in the depres- 

 sion above it. From the lateral lunate surface to the inner wall of 

 the brain-case above the occipital condyle is 2\ centim. 



Finally there remains the interior cavity of the skull which held the 

 brain (PI. XXVII. fig. 6). This does not present any great contraction 

 in the auditory region. Its extreme width behind is 17 millim., where 

 the auditory bones bulge inward after the manner of Crocodiles. The 

 transverse width of the brain- case is thus reduced to 13 millim. ; but 

 at the same time its height increases from 14 millim. behind to about 

 2\ centim. in the region of the auditory prominences, though the 

 extreme height of the brain-case is somewhat in advance of this 

 point, where it becomes 3 centim. The width continues to increase 

 from behind forward to the parieto-frontal suture. It is greatest 

 in the upper third of the outline of the brain, where it amounts to 

 22 millim. A bone which in a bird might be regarded as the ali- 

 sphenoid, which lies above the sphenoid, appears to meet the parietal 

 and exoccipital by a well-defined suture, visible externally and in- 

 ternally, and running obliquely downward and backward. It is 

 difficult to speak with confidence of the limit of this bone on the 

 external surface, since as it extends backwards it is only preserved 

 on the left side. The suture from which it has come away is well 

 defined on the right side. Its anterior border is sharp ; and the 

 external surface is concave from within and outward. This sharp 

 border appears to show that in this Dinosaur the brain-case was not 

 completely closed in front in the middle line. Anterior to the 

 highest point of the upper wall of the brain-case, which lies under 

 the transverse scute-like groove crossing the external surface, the 

 bone makes an angular bend forward; but though there are many 

 little irregularities of outline in the internal surface, there is nothing 

 so important as the bending inward and downward of the lower 

 part of the alisphenoids, which must have made the transverse 

 section of the cerebrum nearly circular at the parieto-frontal 

 sutures. 



Imperfect as this description is, it will suffice to show that we 

 have here a Dinosaur of a type so different from that indicated by 

 the skull referred by Mr. Hulke to Ic/uanodon, as only to be classed 

 in a separate suborder ; and if the base of a skull figured by myself 

 under the name Craterosaurus be, as I believe, also Dinosaurian, 

 that also indicates a subordinal type, and is totally distinct from 

 either of the others. These great differences of skull-structure lead 

 me to suspect that the Dinosauria are a far more important group 



