380 Mr. Bell on a fossil Species of Chelydra. 



layer of the stone when the mass was split. It is necessary to state these 

 particulars^ as a superficial view of the specimen would convey the impression 

 that the under surface is the one exposed. 



The following tabular view of various parts in the recent and fossil species 

 will show at once the distinctions which exist in their relative proportions. 

 It is necessary to bear in mind that the skeleton of Chelydra serpentina, from 

 which the measurements in the second column were taken^ is considerably 

 smaller than the fossil. The dimensions are given in inches and tenths of an 

 inch. 



Fossil. Recent. 



In. Tenths. In. Tenths. 



Length of the dorsal shell 18 11 



Breadth of the dorsal shell at the union with the sternum. . 14 5 9 5 



Breadth of the dorsal shell rather further backwards 15 5 9 5 



Breadth of the sternum at the symphysis, including the ex- 

 pansion of the rami 12 8 5 



Breadth of each ramus of the sternum at the narrowest part 17 5 



Length from the central union of the bones of the sternum 



to the posterior margin of the dorsal shell 10 5 6 



Length of the tail beyond the margin of the shell 12 Q 5 



From a comparison of these details, we find that, taking the dimensions of 

 the dorsal shell as the standard, the following proportional differences exist 

 between the fossil and recent species. The expansion of the upper shell in 

 the fossil is greatest at some distance posterior to its union with the branches 

 of the sternum ; that of the recent is widest at the union. The breadth of the 

 lateral branches or rami of the sternum, in the former, is not less than one 

 eighth of the whole breadth of the sternum ; in the latter it is not more than 

 one seventeenth. These differences, with the much greater comparative 

 length of the tail in the fossil species, warrant me in considering it as a distinct 

 species, to which I propose to give the name Cheli/dra Murcfiisonii, after 

 the distinguished president of this Society, to whose kind interference I owe 

 my possession of it. To his pen I leave the detail of every particular relative 

 to the geological situation of the specimen *, but I think it may not be uninter- 

 esting to state a few facts illustrative of the habitat and manners of the recent 

 species, especially as the organization of the two species is so similar as to 

 preclude the possibility of any considerable difference in their habits. 



The Cheh/dra serpentina is strictly and exclusively an American form. 



There is no approach to it in any species inhabiting the Old World. It is 



found in the southern states of North America, where it is known by the 



name of the snapping turtle. It has also been termed the alligator tortoise, 



* Geological Transactions, Second Series, vol. iii. p. 281. 



