CHELONIA. 11 



less sloping, and the anterior part of the head is more vertically truncate. The orbits 

 are relatively larger, and extend nearer to the tympanic cavity. The frontals (n) 

 enter into the formation of the orbits in rather a larger proportion than in Ghelone 

 mydas. In the Ghelone caouahna* they are wholly excluded from the orbits. 



The trefoil shape of the occipital tubercle is well marked (fig. 4) ; the depression 

 in the basioccipital, bounded by the angular pterygoid ridges, is as deep as in most 

 true turtles (fig. 3) ; the lateral borders of the expanded parietals are united by a 

 straight suture along a great proportion of their extent to the large postfrontals. 



These proportions are reversed in the Podocnemys exjpansa, in which the similarly 

 expanded plate of the parietals is chiefly united laterally with the squamosal and 

 tympanic bones. In other fresh-water tortoises the parietal plate in question does 

 not exist. 



The same evidence of the affinity of the Sheppey Chelonite in question to the 

 marine turtles, is afforded by the base of the skull (fig. 3) ; the basioccipital (l) is 

 deeply excavated ; the processes of the pterygoids (24), which extend to the tympanic 

 pedicles, are hollowed out lengthwise : the palatal processes of the maxillary and 

 palatine bones are continued backwards to the extent which characterises the existing 

 Chelones ; and the posterior or internal opening of the nasal passages, is, in a pro- 

 portional degree, carried further back in the mouth. The lower opening of the 

 zygomatic spaces is wider in the present Sheppey Chelonite, than in Podocnemys 

 expansa. 



The external surface of the cranial bones in the fossil is roughened by small 

 irregular ridges, depressions, and vascular foramina, which give it a wrinkled or 

 shagreen-like character. 



The following are dimensions of the specimen described : 



Inches. Lines. 

 Length of cranium from the occipital condyle ... 2 9 



Breadth of cranium across the malars (26) .... 2 7 



Antero-posterior diameter of orbit ..... 1 



The lower jaw, which is preserved in the present fossil, likewise exhibits two 

 characters of the marine turtles ; the dentary piece (32), e. g. forms a larger proportion 

 of the lower jaw than in the land or fresh-water tortoises. The joint of the rami is 

 completely obliterated at the symphysis, which is not longer or larger than in Chelone 

 mydas. 



The species represented by this fossil, which is preserved in the British Museum, 

 and by a very similar one in the Hunterian Collection, is selected for the first of the 

 Eocene Chelonians to be described in the present Monograph, because it is one of the 

 few with which the characters of the carapace and plastron can with certainty be 

 associated with those of the cranium. 



* Ossem. Fossiles, torn, v, pt. ii, pi. xi, fig. 2. 



