CHELONIA. 23 



The external process of the hyposternal {ps) is relatively much narrower than in 

 the Chel. hreviceps (PI. 16, fig. 2), and, a fortiori, than in C'/«?/. lonfficeps (PI .13, fig. 2). 

 The straight transverse suture by which the hyosternals and hyposternals of the 

 same side are joined together, is much shorter than in the other fossil Chelones ; and is 

 similar in extent to that in CM. mi/das ; but the following differences present themselves 

 in the plastron of the Chelone convexa, as compared with that of the Chelone mi/das. 



The median margin of the hyosternals forms a gentle curve, not an angle : that 

 of the hyposternals is likewise curved, but with a sliglit notch. The longitudinal ridge 

 on the external surface is nearer the median margin of the hi/osteriiah and huposternah 

 and is less marked than in the Chelone longiceps ; especially in the hyposternals, which 

 are characterised by a smooth concavity in the middle of their outer surface. 



Tlie suture between the liyosternals and hi/posterHals is nearer to the external, 

 transverse, radiated process of the hyposternals. The median vacuity of the sternal 

 apparatus is elliptical in the Chel. convexa, but square in the Chel. mydas. 



The characteristic lanceolate form of the episternal bone («) in the genus Chelone, 

 is well seen in the present fossil. The cntosternal element of the plastron is sub- 

 circular, or lozenge-shaped ; and generally broader than it is long in the Emydians. 



The true marine character of the present Sheppey Chelonite, so well given in the 

 carapace and plastron, is likewise satisfactorily shown in the small relative size of the 

 entire femur (65) which is preserved on the left side, attached by the matrix to the left 

 xiphisternal. It presents the usual form, and slight sigmoid flexure, characteristic of 

 the Chelones ; it measures one inch in length. 



In an Emys of the same size, the femur, besides its greater bend, is one inch and a 

 half in length. 



A Chelonian cranium from Sheppey, two inches five lines in length, in the museum 

 of Professor Bell (PI. 24, fig. 4), and a second of the same species from the same locality, 

 two inches nine lines in length (PI. 25, figs. 1, 2, and 3), in the museum of Fred. Dixon, 

 Esq., F.G.S., belong to the same species, and differ from the cranium of the Chelone 

 breviceps, in the more pointed form of the muzzle, and the less rugose character of the 

 outer surface of the bones ; they equally differ from the Chelone longiceps in the less 

 produced, and less acute muzzle, and the more rugose surface of the bones. The 

 parietals (7) are bounded anteriorly by a semicircular line, not by a semioval one, as 

 in Chel. longiceps, or by an angular one, as in Chel. breviceps. The frontals (11) 

 enter into the formation of the orbits, as in both the foregoing species. The orbits 

 are subcircular, as in Chel. longiceps, not subrhomlwidal with the angle rounded off, 

 as m Chel. breviceps. The postfrontals (12) are large, and forma slight projection 

 at the back part of the supraorbital ridge. The tympanic cavity is larger in 

 proportion than in the Chelone longiceps. The palate is traversed by a deep median, 

 longitudinal groove, between which and the shallower grooves on the inner sides of 

 the alveolar borders, are two well-marked, diverging, longitudinal prominences. The 

 bony palate is longer than in Chelone breviceps, shorter than in Chel. longiceps. 



