8 BRITISH FOSSIL REPTILES. 



surface posteriorly for the intervertebral substance and a small part of the succeeding 

 centrum, where a slight expansion of the everted border of the articular surface is 

 the sole indication of such hsemapophysial junction. In the terminal vertebrae these 

 surfaces with the hsemapophyses have disappeared, and the centrum, now showing a 

 compressed form, supports only a contracted, anchylosed, seemingly exogenous neural 

 arch, which finally disappears. 



The following are transverse diameters of the centrum in different regions of the 

 spine, in the specimen, 8 feet 9 inches long, of Plesiosaurus doUchodeirus, figured in 

 Tab. I : 



In. lines. 

 Tenth cervical vertebra ....... 1 



Middle dorsal ditto 2 



Tenth caudal ditto 13 



Cranial characters (Tabs. II and III). 



The skull in this skeleton presents, what is rare, the side or profile view (Tab. II) 

 like that of the succeeding anterior cervicals. Its upper part is much injured. The 

 following bones are recognisable : — mastoid 8, tympanic 28, squamosal 27, malar 26, 

 maxillary 21, premaxillary 22; the end of the long pterygoid is seen at 24, abutting 

 against the lower end of the tympanic. But little of the composition of the mandible 

 is discernible ; the tightly closed jaws show the extent of the interlocking of the 

 long, slender, curved, and sharp-pointed teeth. 



Of those of the lower jaw the crowns of upwards of twenty may be traced ; the 

 longest occupying the middle three fourths of the series, and the largest of these being 

 the foremost. In some parts of the series two teeth pass into the same dental inter- 

 space of the opposite jaw. 



The admirably wrought-out specimen figured in Tab. Ill, fig. 1, exhibits the upper 

 surface of the somewhat crushed skull. Of the basi-occipital a part of the upper surface 

 (l) and of the single median convex condyle is shown. The exoccipitals (ib., 2) preserve 

 their connection with the lateral and upper parts of the basi-occipital, and show the 

 . surfaces — seemingly sutural- — from which the superoccipital (ib., 3) has been displaced. 

 These surfaces (2, 2) are thick' and triangular; they are parallel with the midiMe of the 

 foramen magnum, the lower half of which is formed by the basi- and ex-occipitals. 

 From the outer and back part of the exoccipital the paroccipital process (4, 4) is 

 continued ; of subtriedral form, long, slender, and tapering to a thin I'ounded apex : 

 the outer side appears to be sutural, and that of the left side is applied to the tympanic 

 (ib., 28) : the length of this process is 8 lines. The breadth of the occiput, outside 

 of the exoccipitals, is 1 inch .'j lines ; that of the foramen magnum is 6 lines. That the 

 rough triangular upper surfaces of the exoccipitals are natural, not the result of frac- 



