REMAINS OF A BRITISH FOSSIL ZEUGLODON. 431 



frontal bone, is well seen, and on the inferior margins are two small 

 bones which are in the position of alisphenoids ; posteriorly the bone 

 is very imperfect ; on the under side it is traversed by a groove 

 which widens posteriorly and becomes part of the cerebral cavity. 

 The parietal rests upon and encases the frontal bone, much in the 

 way seen among Ichthyosaurs ; so that while in the fragment mea- 

 suring 6^ inches in length 4| inches are occupied externally by the 

 parietal, on the cerebral surface only 2\ inches are occupied by that 

 bone. The sides of the parietal bone are nearly vertical, and nearly 

 parallel, measuring 2 inches from side to side in front, where they are 

 about 1| inch high. Above this height the sides become rounded, and 

 converge towards the middle line into an elevated mesial keel, which 

 rises higher the further it is prolonged backward ; so that while the 

 bone as it stands on its sutural base is but little over 2 inches high 

 in front, it becomes, where fractured behind, nearly 3| inches high. 

 In front the ridge is rounded ; but the last 1J inch of it preserved 

 becomes flattened horizontally and widens posteriorly, being at the 

 fracture J inch wide. Concomitantly with the formation of this 

 ridge the bone appears to widen out from side to side behind, and 

 the lateral inclined halves of the upper surface exchange their 

 rounded outlines for a sloping flattened surface. In front the ridge 

 dies away just behind the suture. The suture is deep and well 

 marked ; it is somewhat irregular, and penetrates back into the 

 parietal in the form of an inverted W with long outer arms, 

 which on the shoulder of the side of the bone contribute to form a 

 similar unin verted figure, the outer arm of which, in an irregular 

 line, is prolonged downward and backward at an angle of about 45° 

 with the basal outline of the bone ; so that the parietal extends a 

 less distance along the side than along the superior surface, where 

 its termination is bifid. 



The Frontal Bone. 



The sutural end of the frontal necessarily corresponds closely 

 with the parietal ; but as the sides of the bone are similarly flat and 

 vertical, and the superior surface is horizontal, it results that the 

 section of the bone at the fracture is nearly quadrate, being rather 

 higher than wide. The frontal bone is also notched on the under- 

 side with a nearly quadrate olfactory canal about half an inch in 

 section. The angles between the lateral surfaces and superior sur- 

 face become elevated ; and where it is fractured anteriorly the bone 

 appears to be widening outward from side to side. The horizontal 

 surface of the frontal continues forward the anterior depression in 

 height of the parietal, and at its anterior termination the frontal is 

 but H i ncn high. 



Thus the portion of the frontal preserved has extremely thick 

 walls. The walls of the skull scarcely become thinner in the pari- 

 etal region ; for the groove which traverses the bones only slowly 

 widens and deepens. Its surfaces are nearly flat, and, except that 

 it is relatively deeper, it corresponds closely in form with the external 



Q. J. G. S. No. 128. 2 h 



