168 BRITISH FOSSIL REPTILES. 



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 length of the carapace of the Chelone mi/das is about nine times that of the 

 sixth marginal plate, whence I calculate the length of the carapace to which the 

 marginal plates here described belonged to have been about fourteen inches. 



The following admeasurements will show the different proportions of the marginal 

 plates of the present specimen as compared with the corresponding ones of a Chelone 

 mydas of similar general size : — 



Length of the series of five plates in a straight line 

 Breadth of the upper surface of the third (fifth) 

 Interspace of costal depressions 



Thus the marginal plates of the chalk turtle, besides being more concave, are 

 broader in proportion to their length, or antcro-posterior diameter. In these respects 

 they correspond with the form of the marginal plates in the Chelone Benstedi, but 

 more evidence must be had, before these large fossil marginal plates can be referred 

 to a larger and older specimen of the species. 



There are other two marginal plates imbedded in the same portion of chalk, with 

 their upper, smooth, slightly concave surfaces exposed ; and the toothed or sternal 

 extremities of three of the vertebral ribs, which by their length and size also prove 

 this specimen to be a Turtle. One of these fragments of rib measures 5^ inches, and 

 the expanded plates developed from each side of its upper surface are concave on 

 their exterior surface, which is flat or slightly convex in Chelone mi/das. 



A separate portion of chalk from the same pit contains the scapula and its acromial 

 branch or anchylosed clavicle, with the articular surface which joins with the coracoid 

 and humerus. The angle at which the scapula and clavicle meet is more open in Chelone 

 than in Emys or Chelys : the present specimen presents the same angle as in the 

 Maestricht Chelone figured by Cuvier,t in which it is rather more open than in the 

 recent species of turtle. A broad, thin, slightly concave plate of bone appears, by 

 the radiation of the fine striae at its under part, to represent the expanded parietal 

 bone of the cranium. 



* 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 difi'erence is less than in the 

 present fossU species. 



f Ossem. Foss., torn, v, part ii, pi. xiv, fig. 5. 



