CHELONIA. 171 



In Lower Jurassic rocks no Chelonian remains are known, 

 except possibly some epidermal shields from the Stonesfield 

 Slate in the neighbourhood of Oxford. In the Upper Jurassic 

 all the genera hitherto discovered are so closely similar to those 

 now existing, that as yet they scarcely unite the three sub-orders 

 which are completely differentiated in the modern fauna. The 

 only known skulls are entirely toothless and nearly all exhibit a 

 roof over the temporal muscles like that of the existing Chelone. 

 The digits comprise the small number of phalanges so generally 

 characteristic of the Chelonian order. The carapace and plastron 

 are fully formed, and always bear marks of the presence of epi- 

 dermal shields. In fact, so far as present evidence permits of 

 judgment, it seems that since early Cretaceous times, degenera- 

 tion of the carapace and plastron in certain types and elonga- 

 tion of the phalanges in the truly marine forms, are the only 

 conspicuous marks of evolution in the order. The enlargement 

 of the vacuities in turtles like Chelone, and the degeneration of 

 the entire armour of the leathery turtles (Sphargis and its 

 allies) have happened in the interval. The sub-order of Tri- 

 onychia, destitute of epidermal shields, seems also to have 

 arisen. The development of the neural bones has become more 

 irregular in some groups, and the occurrence of mesoplastral 

 elements seems to be now less common than it was in Mesozoic 

 times. 



It will suffice to enumerate the characters of three of the 

 best-known Jurassic genera, the first two perhaps Cryptodiran, 

 while the third may be a primitive Pleurodiran. 



Eurysternum. A genus known by individuals in many stages of 

 growth from the Lithographic Stone (Upper Jurassic) of Bavaria and 

 France. The temporal fossae in the skull are extensively roofed. Large 

 vacuities persist throughout life in the plastron and between the extremi- 

 ties of the costal bones. There are distinct marks of epidermal shields 

 both on the carapace and plastron. The tail is short. The limbs are 

 completely known and distinctly those of a land-animal, the digits five in 

 number on each foot, with short phalanges and terminal claws. The 

 pelvis is not fused either with the carapace or with the plastron. . The 

 typical species is E. wagleri, of quite small size. 



Chitracephalus. A genus known only by one nearly complete skele- 

 ton (C. dumoni) from the Wealden of Beruissart, Belgium. The cranium 

 is much elongated and depressed, with an extremely short facial region, 



