376 Geological Society : Prof. Owen's Description 



which are narrow in proportion to their length, than in any of the 

 previously described species ; hut is more conspicuously distinct by 

 the form of the 6th and 8th vertebral plates, which support a short, 

 sharp, longitudinal crest. The middle and posterior part of the first 

 vertebral plate is raised into a convexity, as in the Chel. longiceps, 

 but not into a crest. 



The keeled structure of the sixth and eighth plates is more marked 

 than in the fourth and sixth plates of Chelone Mydas, which are 

 raised into a longitudinal ridge. 



The characters of the carapace are then minutely described. 



Sufficient of the sternum is exposed in the present fossil to show, 

 by its narrow elongated xiphisternals, and the wide and deep notch 

 in the outer margin of the conjoined hyo- and hyposternals, that it 

 belongs to the marine Chelones. 



The xiphisternals are articulated to the hyposternals by the usual 

 notch or gomphosis ; they are straighter and more approximated 

 than in the Chel. Mydas ; the external emargination of the plastron 

 differs from that of the Chel. Mydas in being semicircular instead 

 of angular, the Chel. subcristata approaching, in this respect, to the 

 Chel. breviceps. 



The shortest antero-posterior diameter of the conjoined hyo- and 

 hyposternals is two inches seven lines. The length of the xiphi- 

 sternal two inches six lines. The breadth of both, across their 

 middle part, one inch three lines. 



The name proposed for this species indicates its chief distinguish- 

 ing character, viz. the median interrupted carina of the carapace, 

 which may be presumed to have been more conspicuous in the horny 

 plates of the living animal than in the supporting bones of the fos- 

 silized carapace. 



6. Chelone planimentum. — This species is founded on an almost 

 entire specimen of skull and carapace of the same individual, in the 

 museum of Prof. Sedgwick; on a skull and carapace belonging to 

 different individuals, in the museum of Prof. Bell ; and on a carapace 

 in the British Museum ; all of which specimens are from the London 

 clay at Harwich. 



The skull resembles, in the pointed form of the muzzle, the Chel. 

 longiceps of Sheppey, but differs in the greater convexity and breadth 

 of the cranium, and the great declivity of its anterior contour. 



The great expansion of the osseous roof of the temporal fossae, and 

 the share contributed to that roof by the. post-frontals, distinguish 

 the present, equally with the foregoing Chelonites, from the fresh- 

 water genera Emys and Trionyx. In the oblique position of the 

 orbits, and the diminished breadth of the interorbital space, the pre- 

 sent Chelonite, however, approaches nearer to Trionyx and Emys than 

 the previously described species. 



Its most marked and characteristic difference from all existing or 

 extinct Chelones is shown by the greater antero-posterior extent and 

 flatness of the under part of the symphysis of the lower jaw, whence 

 the specific name here given to the species. 



Since at present there is no means of identifying the well-marked 



