ON BRITISH FOSSIL REPTILES. 1/7 



marginal plates of the present specimen as compared with the corresponding 

 ones of a Cheloiie mi/das of similar general size : — 



Length of the series of five plates in a straiglit line 

 Breadth of the upper surface of the third (fiftii) . 

 Interspace of costal depressions 



Thus the marginal plates of the chalk turtle, besides being more concave, 

 are broader in proportion to their length, or antero-posterior diameter. In 

 these respects they correspond with the form of the marginal plates in the 

 Chelone Benstedi, and most probably belong to a larger and older specimen of 

 the same species. 



There are two other marginal plates imbedded in the same portion of 

 chalk, Avith their ujiper, smooth, slightly concave surfaces exposed; and the 

 toothed or sternal extremities of three of the vertebral ribs, which by their 

 length and size also prove this specimen to be a turtle. One of these frag- 

 ments of rib measures 5h inches, and the expanded plates developed from 

 each side of its upper surface are concave on their exterior surface, which is 

 flat or slightly convex in Cheloiie mydas. 



A separate portion of chalk from the same pit contains the scapula and its 

 acromial branch or anchylosed clavicle, with the articular surface which joins 

 with the coracoid and humerus. The angle at which the scapula and clavicle 

 meet is more open in Chelone than in Emys or Chelys : the present specimen 

 presents the same angle as in the Maestricht Chelone figured by Cuvier *, in 

 which it is rather more open than in the recent species of turtle. A broad, 

 thin, slightly concave plate of bone appears, by the radiation of the fine 

 striaj at its under part, to represent the expanded parietal bone of the 

 cranium. 



The carapace of the turtle to Avhich the fragments above described be- 

 longed, must have been nearly if not quite two feet in length. 



Eocene Tertiary Chelones. — Although both the leading divisions of fresh- 

 water Chelonians are represented in the Eocene tertiary formations of Great 

 Britain, the one by the Emys testudiniformis, the other by the Plaiemys or 

 Hydmspis Bullockii, the Chelonian Reptiles from the London clay of Shep- 

 pey and Harwich are for the most part true turtles, or species of the genus 

 Chelone. Already good evidence of at least five distinct species have been 

 obtained from these localities, and it is probable that others remain to be 

 discovered ; they are generally of smaller size than the species which are now 

 restricted to warmer or intertropical latitudes, and differ from those species, 

 as well as from each other, by well-marked characters afforded by the skull, 

 the carapace, and the plastron. 



Chelone longiceps — The most common species, Chelone longiceps\, is di- 

 stinguished by very interesting modifications both of the cranium and osseous 

 buckler, by which it approaches more nearly to the freshwater Chelonians 

 than do any of the existing species of Chelone. In the prolongation of the 

 conical rostrum and osseous palate, the skull of this species resembles that of 

 a Trionyx, but the tem.poral fossae are covered by a roof of bone having the 

 characteristic anatomical structure of the true Chelones. The buckler is broader 

 in proportion, and both carapace and plastron are more completely ossified 

 than in recent turtles ; thus both the hyosternals and hyposternals are broader 



* Ossein. Foss., torn. v. part ii. pL xiv. fig. 5. 



t The characters of this and the other species of Chelone from the Loudon clay forma- 

 tion are detailed in ray Memoir on that subject read before the Geological Society, Dec. 1, 

 1841, 



184:1. N 



