26 BRITISH rOSSIL REPTILES. 



The osseous roof of the temporal fossae, and the share contributed to that roof by 

 the postfrontals (PI. 18, figs. 1 and 2, 12), distinguish the present, equally with the 

 foregoing Chelonites, from the Emys {Podocnemys) eccpansa, and, a fortiori, from other 

 genera and species of the fresh-water families {Emydidce and TrionicidcB) . 



In the oblique position of the orbits (fig. 2), and the diminished breadth of the 

 interorbital space (fig. 1), the present Chelonite, however, approaches nearer to Trionyx 

 and Emys than do the previously-described species. But the sides of the face converge 

 more rapidly towards the muzzle. Its most marked and characteristic difference from 

 all existing Chelones is shown by the greater antero-posterior extent, breadth, and 

 flatness of the under part of the symphysis of the lower jaw (fig. 3), whence the specific 

 name here given to the species. The posterior border of the symphysis is defined 

 by a regular semicircular curve, and the rami of the jaw have completely coalesced. 



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

 the skull is here described, with the Chelonite figured in the frontispiece to Woodward's 

 ' Synoptical Table of British Organic Remains,' and alluded to, without additional 

 description or characters, as the Chelonia ITarvicensis, in the additions to Mr. Gray's 

 ' Synopsis Reptihum' (p. 78, 1831); and since the extensive deposit of Eocene clay 

 along the coast of Essex, like that at the mouth of the Thames, contains the relics of 

 more than one species of ancient British turtles,* I prefer indicating the one here 

 established by a name having reference to its peculiarly distinguishing character, 

 rather than to associate arbitrarily the skull, which gives the true specific distinction, 

 with the ill-defined carapace to which the vague name of Harvicensis has been applied; 

 more especially as the fossil carapace to which the present skull more probably 

 belongs, from the circumstance under which it was discovered, also presents well- 

 marked, and readily-recognisable specific characters. 



This carapace (PI. 19) is also contained in the museum of Professor Sedgwick, and is 

 understood to have formed part of the same individual turtle as the skull (PI. 1 8) on 

 which the species, Chel. planimentum, was founded. 



In general form this carapace differs from that of the existing Chelones, in being 

 less contracted and pointed posteriorly than in the Chelone mydas and CJiel caouanna, 

 and more contracted posteriorly than in the Chel. iinhricata. In the proportion which 

 the pleurapophyses (true ribs), bear to the superimposed costal plates, (yV4 — 8) it 

 resembles Chelone mydas, and Chelone caouanna, more than it does the Chel. imbricata. 

 But the pleurapophyses are more prominent and distinct from the costal plates 

 throughout their entire length, than in the Chel. viydas or Chel. caouanna, and present 

 an obtuse angular ridge towards the cavity of the abdomen. 



The five posterior pairs of ribs of the carapace {pl\ — jih) are preserved, with part 



* Sir C. Lyell alludes to the Chelonites of Harwich in his ' Elements of Geology :' " This formation is well 

 seen in the neighbouring clifls of Harwich, where the nodules contain many marine shells, and sometimes 

 the bones of Turtles." (Vol. ii, p. 337.) 



