642 . PKOF, H. G. SEELET ON THE SKTJLI* OE 



uncertain distance, but apparently at least as far as the region of the 

 exterior nares. In the median line of the base of the skull are seen 

 the displaced basioccipital (bo), the basisphenoid (6s), and presphenoid 

 (^s), the latter two being anchylosed together. The basioccipital 

 has a somewhat flattened hemispherical articular surface, rough with 

 concentric markings of attachment, and defined, as usual, by a mode- 

 rately deep vertical and inferior groove. It is about 3 inches wide. 

 Below and in front of this part of the bone is a smooth non-articular 

 surface, which is concave in the middle and, where widest, extends 

 the width of the bone to nearly 4 inches. Its length is rather over 

 3 inches, and, as is usually the case with Liassic Ichthyosaurs, it has 

 the anterior margin, where it abuts against the basisphenoid, convex 

 from side to side. The basisphenoid bone (bs) is exposed but little. 

 It appears to have been about 2J inches long, with a strong elevated 

 ridge in the middle of the base, on each side of which the surface is 

 concave. This ridge is prolonged forward into the sword-shaped 

 presphenoid bone (ps) with which it is anchylosed. On each side 

 of the presphenoid the anterior margin of the basisphenoid is deeply 

 concave ; the posterior lateral portions of the basisphenoid are over- 

 lapped by wide inward expansions of the pterygoid bones. The 

 presphenoid slightly tapers as it extends forward, is compressed from 

 side to side, preserves its inferior keel, and is at last received between 

 a pair of bones anterior and interior in position to the pterygoids. 

 It has a length anterior to the basisphenoid of about 8| inches. 



External to these median bones there appear to be four pairs of 

 lateral bones. The largest of these are the pterygoid bones (p), 

 situate behind ; the bones external to the pterygoids are two pairs — 

 a slender anterior pair, which I regard as the palatines (pn) ; and a 

 larger pair external to the palatines and extending partly along 

 them and the pterygoids, but well separated from the malar, which 

 I regard as the transverse bones (t) ; while interior to the pterygoids 

 and the palatines and abutting against the presphenoid is another 

 pair of slender bones which, from their position, I regard as the 

 vomers (v). 



The arrangement of the bones in this genus is very different from 

 that in the genus in the Inferior Oolite of Caen, as shown by a skull 

 in the British Museum, unless the bones which here have every 

 appearance of separate existence, and which I regard as the vomers, 

 should be an accidental condition marking a generic difference. 

 Moreover the pterygoid in the Caen fossil, on the left side, consists 

 of two separate osseous elements. The pterygoid bones (p) have 

 their inner margins concave. They do not appear to have met 

 posteriorly in the median line, though from the displacement of the 

 basioccipital this cannot be stated with absolute confidence. They 

 partly enclose the long pterygoid vacuities already alluded to, which 

 are divided by the presphenoid. These vacuities are margined in 

 front by the bones interior to the pterygoid which I have regarded 

 as vomerine. The pterygoids are deeply notched out on the pos- 

 terior aspect by a concave border, and in the external posterior 

 half have a concave outline. Between these two concavities is 



