170 REPOKT— 1841. 



structure in the present skull, though hitherto unknown in the genus Chelone, 

 has been met with in the Platemydian subgenus, Chelodina, the cranium of 

 which, in other respects, closely conforms to the ordinary Emydian type, and 

 has not the temporal fossas protected by bone, as in tW ' Emys {Podocnemis) 

 expansa*. 



Chelone obovata, nob. — The most compli 3 and beau*'*"" I specimen of a 

 fossil turtle, from secondary strata, that I liav&'j <^ set one from the 



estuary limestone formation of the Isle of Purbeck, in 'ihe CTolTection of Chan- 

 ning Pearce, Esq. of Bradford, Wiltshire. 



This species differs from the Chelone Benstedi of the chalk, from the Gla- 

 ris Turtle, and from all the well-determined Eocene species in the form of 

 the carapace, which, although obtusely pointed behind, begins to contract to 

 that extremity only at its posterior third part ; it gradually widens through 

 the two anterior thirds of its extent, and is broadest at the junction of the 

 fifth and sixth ribs ; the contour being obversely ovate, or with the broader 

 end turned downwards. This modification of form arises, not from the supe- 

 rior length of the fifth and sixth pairs of ribs, but f;'om the breadth of their 

 sternal appendages, called marginal plates. The internal surface of the cara- 

 pace is exposed to view, and is shallower than in any other Chelone ; resem- 

 bling in this character the Trionyx and Tretosternon ; the margins of the 

 carapace are slightly bent upwards, as in some Emydes. 



The first vertebral plate is shorter antero-posteriorly, and less deeply emar- 

 ginate anteriorly than in the Maestricht Chelone f, and the first pair of ex- 

 jjanded ribs is narrower. This well-marked difference fortunately occurs in 

 the only parts in which a comparison could be established with the Tui'tles 

 from a stratum so nearly contemporaneous with the Purbeck beds. The first 

 rib is straight, and rather narrower in proportion than in the Eocene Turtles ; 

 in Chelone mydas this rib is the broadest of the series. It very gradually con- 

 tracts into its dentiform extremity, which on the left side appears to have been 

 separated by a narrow membranous space from the anterior marginal plates, 

 but not on the right side. The second rib is broader in proportion to the 

 first than in any other species of Chelone, recent or extinct ; more so even 

 than in the Trionyx, in which the first rib is narrower than the second. The 

 third and fourth ribs are the broadest. 



In. Lin. 

 The length of the expanded part of the fourth rib is . 3 

 The breadth of the expanded part of the fourth rib . 1 5 

 The length of the dentiform extremity 10 



The rest gradually diminish in length and breadth. They are all as flat 

 upon the under surface as in Chelone mydas, presenting a great contrast to 

 the Harwich species, Chel. planimentum, in this respect. 



The median row of vertebral plates, after the first, are as narrow as in most 

 Chelones, and appear, as far as their form can be traced from an inside view, 

 to resemble those of the Chel. mydas and Chel. longiceps, but to have been 

 narrower than in the extinct Eocene Turtles : the length of the fourth ver- 

 tebral plate in the Chel. obovata, for example, is 1 inch 3 lines, its greatest 

 breadth is 6 lines. The eleventh six-sided plate, which resembles a triangle, 

 with truncated angles, and is wedged between the last pair of ribs, is here 

 divided by a transverse suture into two nearly equal parts. The twelfth plate 



* The different proportions in which the cranial bones, especially the post-frontals, enter 

 into the formation of the bony covering of the temporal fossa; in the Emys expaiisa, serve to 

 distinguish the skull of a Chelone from that of this exceptional example of an Emydian with 

 the temporal fossae covered by bone. 



t Cuv. Ossem. Foss,, tom. v. pt. 2. pi. xiv. Tortues, fig. 1 and 2. 



