166 REPORT — 1841. 



like portion of the rib from the distal end of the expanded part, are essential 

 distinctions. 



The entire and rounded terminal margins of the truncated and expanded 

 extremities of the ribs, beyond which there is not the slightest trace of pro- 

 jecting tooth-lilce processes, strongly indicates that the marginal plates were 

 either wanting or rudimental, as in the genus Cryptopus, 



This fossil is from the Purbeck limestone. 



In the collection of Mr, Bowerbank is preserved the left half of the plas- 

 tron of the same species of freshwater tortoise, from the Purbeck limestone, 

 at Swanage, equal in size and probably of the same species as the preceding, 

 but with fainter impressions on the external surface. This Chelonite includes 

 the hyosternal, the hyposternal, and a considerable part of the xiphisternal 

 bones, but wants the extremity of this bone. It is a very remarkable and 

 characteristic fossil, chiefly on account of the great extent of the lateral wall, 

 which is continued outwards, in the same plane with the rest of the plastron, 

 as in the Emydian subgenus Platysternon, Gray, accompanied by an unusual 

 width of the notches, anterior and posterior to this wall, for the emergence of 

 the fore and hind-feet. The length of this fossil, taken along the median su- 

 ture, is 13 inches; the breadth of the sternum along the median transverse 

 suture must have exceeded 12 inches. The antero-posterior extent of the 

 contracted part of the lateral wall is 7 inches ; that of its expanded outermost 

 part 9 inches ; the antero-posterior diameter of the hyposternal bone 4 inches 

 4 lines ; extent of transverse suture between this and the xiphisternal 3 inches. 

 The outer and anterior angle of the xiphisternal has a shallow angular notch 

 which receives a corresponding process of the hyposternal : the median mar- 

 gin of the hyposternal has a semicircular piece cut out of it just where it joins 

 the hyosternal : the bone gradually narrows off to the edge of this emargi- 

 nation, which is exposed, by a careful removal of the matrix, without any 

 trace of fracture of the bone. If it be, as it seems, a natural structure, then 

 the centre of the sternum must have presented an elliptical vacuity, closed by 

 membrane or cartilage of nearly two inches diameter, situated immediately 

 behind the transverse suture uniting the hyo- and hyposternals. Such an ap- 

 proximation to the Trionyces and Chelones presented by an extinct species, 

 which from the extensive lateral union by a continuous bony plate of the side 

 of the plastron with the carapace, and from the complete ossification of the 

 latter is essentially an Emydian species, forms a very interesting transitional 

 modification, especially if it be combined, as there seems good reason to be- 

 lieve, with the sculptured surface of a remarkably flattened carapace, such as 

 the Chelonite, in Sir P. Egerton's collection, from the same stratum and loca- 

 lity presents. 



The sternum, like the carapace above mentioned, is impressed by the mar- 

 gins of distinct scutes. 



The transverse line bounding the second sternal scute has the same rela- 

 tive position as in the Testudo Schweigeri and Platemys planiceps. The two 

 succeeding sternal scutes have a more equal antero-posterior extent than in 

 those species. The impression commences at the median line, nearly an inch 

 in advance of the transverse suture and three inches behind the second trans- 

 verse scute, and describes a slight curve which is convex towards the anterior 

 part of the plastron. The third scutal line commences from the middle of the 

 median emargination, and instead of running parallel with the preceding line, 

 as in the Tortoises and ordinary Emydes which I have examined, it inclines 

 backwards as it passes outwards, and terminates at the middle of the posterior 

 lateral emargination. The fifth scutal line is oblique, as in the Emydians 

 generally, and here therefore runs parallel with the fourth line at a distance 



