176 REPORT — 1841. 



the marginal plates, tlie non-development of the lower margin of any of these 

 plates for a junction Avith the plastron, the narrow elongate form of the ver- 

 tebral plates, and the broad vertebral scutes, collectively and separately mili- 

 tate ; whilst in all these modifications the Turtle from the Chalk so closely 

 corresponds with the true Chelones, that I cannot hesitate to refer it to the 

 marine family of the order. , 



From the breadth of the xiphisternals in the remains of this species first 

 described by me, I was induced to suppose that a new subgenus ( Cimochelys) 

 of marine Turtles was thereby indicated, having a closer affinity to the Emydes 

 than the typical species ; and the same affinity seems to be shown by the 

 more regular elliptical form of the carapace of Mr. Bensted's beautiful spe- 

 cimen. The structure of the cranium, when this desirable part of the skeleton 

 is discovered, may confirm the propriety of the subgeneric distinction ; but 

 the numerous decided marks of closer affinity to Clielone leave no alternative 

 than to regard the fossil species of the chalk as a member of that genus. 



It difi'ers from all known species, especially the sub-carinated species of 

 Sheppey, in the form of the carapace, which is more truly elliptical than in 

 any other species with which I am acquainted. 



I have been favoured with. the opportunity of inspecting portions of the 

 skeleton of a large Chelonian obtained by Mrs. Smith, of Tonbridge Wells, 

 from the lower chalk at Burham, Kent, and skilfully relieved from iheir 

 mineral bed by that lady. The principal bones consist of two series, one 

 containing five, the other four, of the marginal plates of the carapace, in 

 natural connection, and from that part of the margin where they receive the 

 extremities of the vertebral ribs. These marginal plates in Chelone mydas 

 are three-sided, and have two terminal surfaces by which they are united, 

 suturally, to one another : of the three free surfaces, the one, directed towards 

 the interior of the body, is characterized by a deep depression for the recep- 

 tion of the tooth-like extremity of the rib ; the two otlaer (upper and under) 

 surfaces meet at an angle, which is produced at certain parts to form the 

 marginal dentations of the lateral and posterior parts of the carapace in that 

 species of turtle, but is more open and obtuse in the marginal plates at 

 the anterior part of the carapace. In the fossil the marginal plates have the 

 general characters of those of the genus Clielone, but differ from those of the 

 Chelone mydas, in being more concave on the central or perforated side, and 

 they are also concave at the upper side, and in a slighter degree at the under 

 side ; these sides likewise meet at a more acute angle, and this angle is pro- 

 duced into a sharper and more continuous ridge : but this ridge subsides at 

 one end of the series of five plates, and the upper and under sides gradually 

 meet at a more open angle, which is rounded off in the first of the series. 

 This plate, therefore, answers to the third marginal plate in the Chelone 

 mydas, or that which receives the end of the first expanded vertebral rib ; 

 and the remainder, therefore, to the fourth, fifth, sixth, and seventh marginal 

 plates : now these are precisely the marginal plates in the Ernys, which have 

 their inferior margins developed inwards and articulated by suture to the 

 lateral wall of the carapace : but these margins not being so developed or 

 terminated in the present fossil, but, on the contrary, being inferior to the 

 upper margin in breadth*, and terminating like that margin in a blunted 

 edge, prove the present Chelonite to belong, like the smaller Chelonite from 

 the same chalk-pit already described, to the marine genus Chelone. 



The following admeasurements will show the different proportions of the 



* The upper margin, which is distinguished by a slight notch where the costal groove leads 

 to the pit, is broader than the lower one, in these plates of the Chelone mydas ; but the dif- 

 ference is less than in the present fossil species. 



