28 FOSSIL REPTILIA OF THE LONDON CLAY. 



fig. 1, with T. XI, fig. 3). With regard to the comparative anatomy of the bones of 

 the skull, and the pattern of the scutation of the upper surface of the cranium, I regret 

 that the state of the specimen in Professor Bell's collection does not permit the 

 deduction of other distinctive characters which such parts of the cranial organization 

 so satisfactorily afford. A great proportion of the osseous parietes is wanting ; but the 

 cast in the hard matrix of the wide lateral cavities (12, 12), which were over-arched by 

 the expanded postfrontal and parietal bones, indicates the prominence of the postfrontals 

 at the upper and outer angle of the orbits. The orbits (or) appear to have been more 

 ovate and less circular than in the Chelone planimentum ; and the sides of the orbital 

 part of the skull do not converge so rapidly towards the muzzle, but meet at a more 

 acute angle. 



That a second species of turtle, distinct from the Chelone planimentum, has left its 

 remains in the Harwich clay, is very decisively demonstrated by the almost complete 

 carapace in the British Museum, the inner surface of which is represented, on the 

 scale of six inches to a foot, in T. XII. This carapace, both by its general contour, by 

 the relative length of the costal plates to one another, and by their relative breadth to 

 the adherent pleurapophyses beneath, more resembles the carapace of the Chelone 

 imbricata than that of the other known existing species of turtle ; and, as the peculiar 

 characters of the Chelone imbricata are exaggerated, it differs in a proportional degree 

 from the Chelone planimentum. These characters are seen in the great breadth of the pro- 

 minent inferior part of the ribs, and of the free extremity of the rib (pli—plg), as compared 

 with the total breadth of the costal plate. The intervals between the free extremities, 

 where the expanded plate terminates, are not equal to the breadth of the proper ribs ; 

 in the Chelone imbricata they very slightly exceed the breadth of the free ends of the 

 ribs. This character in the fossil, by which it is so markedly distinguished from the 

 Chelone planimentum, and most other species, has suggested the name Chelone crassi- 

 costata, or thick-ribbed turtle, which is proposed for the present species. The last 

 pair of ribs of the carapace (T. XII, pis) are remarkably short and thick, and are 

 curved backwards on each side the broad terminal neural plates which they almost 

 touch. In this character the Chel. crassicostata resembles the Chel. imbricata, and 

 differs from the Chel. caouanna (fig. 2, p. 3), and from Chel. mydas. The subequality 

 of length of the costal plates is another character by which the Chel. crassicostata 

 resembles the Chel. imbricata, and differs from the Chel. mydas, the Chel. caouanna, as 

 well as from the Chel. planimentum. 



In T. XII, as in the other figures, ch is the nuchal plate, ph the first rib of the 

 carapace (the second free pleurapophysis or vertebral rib), ph to ph the remaining 

 ribs of the carapace and costal plates ; 6-9, sio, and py are the terminal neural plates 

 and pygal plate, which, like the nuchal plate, are developed in the substance of the 

 integument, without becoming attached to the subjacent spinous processes of the 

 vertebrae. The debris of the neural arches of the intermediate eight vertebrae of the 



