174 REPORT — 1841. 



The fossil in question consists of nearly the whole carapace, and a con- 

 siderable portion of the plastron, with a coracoid bone. 



The carapace includes all the dorsal or vertebral plates, save the first; the 

 usual number of expanded ribs, viz. eight pairs ; and the entire border of 

 marginal plates, save the three first. In the sternum the hyosternal and hy- 

 posternal bones may be distinguished. The general form of the carapace is 

 elliptical, terminated by a point at the narrower posterior end, Avhich, how- 

 ever, is less contracted than in other Chelones. It is as depressed as in Che- 

 lones generally. To judge from the unniutilated vertebral plates, which are 

 the four last, the carapace appears to have been traversed by a median longi- 

 tudinal crest, from which the sides gently slope with a slight convex curva- 

 ture, as in Chelone mydas. 



The more immediate indications of the close affinity of the fossil to the 

 marine turtles are given by the incomplete ossification and anchylosis of the 

 ribs and sternal bones, the latter being in consequence dislocated from each 

 other ; and more especially by the shape and size of the marginal plates at- 

 tached to the third, fourth, fifth, and sixth ribS; as also by the form and 

 length of the coracoid bone. 



The vertebral plates are as narrow relatively as in the ordinary Chelones ; 

 but their precise form can only be distinguished in the three last. The ninth*, 

 or that to which the eighth rib is in part articulated, is 3 lines in length and 

 2 in breadth ; the tenth expands posteriorly into a triangular form ; both 

 these have their middle part raised into a ridge ; the eleventh plate is sud- 

 denly expanded, with angular sides, which slope away from a median longi- 

 tudinal ridge : this is crossed by a transverse ridge just anterior to the junc- 

 tion of the plate with the median terminal plate of the marginal series, which 

 is convex above and traversed by a median longitudinal furrow. The margins 

 of this plate meet posteriorly at an open angle. The second to the seventh 

 pairs of expanded ribs are joined together only along their vertebral halves. 

 The length of the expanded part of the third rib is 9 lines ; its narrow, 

 tooth-like part, before it reaches the marginal plate, is also 9 lines ; about 3 

 lines of its extremity is inserted into the deep groove of the concave surface 

 of the marginal plate. The width of the interspace between the narrow parts 

 of the third and fourth ribs is 4 lines ; the length of the expanded part of the 

 first rib is IO5 lines; the breadth of the expanded part of the first rib is 8 

 lines ; the length of the narrow end of the rib, clear of the marginal plate, 

 is 3 lines. In the superior bi'eadth of the first rib the Chelone Bensledi agrees 

 with existing turtles, and differs strikingly from the Purbeck species. The 

 last short rib sends, almost directly backwards, a short, narrow, tooth-like 

 process, at right angles to the anterior margin of its sub-triangular expanded 

 part. In Chelone obovata it is extended more nearly parallel with the ex- 

 panded part. 



The marginal plates have the same general uniformity of size which we 

 observe in the existing Chelones ; the posterior ones are not expanded as in 

 the Purbeck Chelone, and in certain Emydes, as Emys serrata. See. ; but the 

 most decisive evidence against the Emydian affinities of the present fossil is 

 afforded by the form and development of the inferior borders of the marginal 

 plates attached to the fourth, fifth, and sixth ribs; for these plates, instead of 

 being expanded and extended inwards to join the hyo- and hyposternals, and to 

 combine with these elements of the plastron in forming the lateral supporting 

 wall of the carapace, are not so much developed in breadth as the same parts of 

 the posterior marginal plates, but form with them an even free border, as in other 

 Chelones; in which not any of the marginal plates are joined with the sternum. 

 * In all Emydes the proportions of this plate are the reverse of those in the fossil. 



