A HISTOEY 



OF 



BRITISH FOSSIL REPTILES. 



CHAPTER I.— Order CHELONIJ. 

 TURTLES, TERRAPENES, AND TORTOISES. 



^ 1. Introdadory BeDiarks on the Ilomolor/i/ of ihe Carapace and Plastron. 



The majority of the Fossil Chelonians, defined or descril^ed in my ' Report on 

 British Fossil Reptiles,'* belonged to the marine division of the order, and as the 

 species of this family depart least from the ordinary reptilian type in the modification 

 of the -bones of the trunk, composing the characteristic thoracic-abdominal case of 

 the order, I propose to commence with them the descriptions of the Fossil Reptilia 

 which form the subject of the present Chapter. 



In order to facilitate the comprehension of the descriptions and figures of the 

 fossil Chelonians, a brief notice is premised of the composition and homologies of the 

 carapace and plastron, or roof and floor, of that singular portable abode, with which the 

 reptiles of the present order have been endowed in compensation for their inferior 

 powers of locomotion or other modes of escape or defence. 



In the marine species of the Chelonian order, of which the Chelone nujdas may be 

 regarded as the type, the ossification of the carapace and plastron is less complete, 

 and the whole skeleton is lighter than in those species that live and move on dry 

 land: but the head is proportionally larger — a character common to aquatic animals, — 

 and being incapable of retraction within the carapace, ossification extends in the 

 direction of the fascia, covering the temporal muscles, and forms a second bony 

 covering of the cranial cavity ; it is interesting to observe, however, that this accessory 

 defence is not formed by the intercalation of any new bones, but is due to exogenous 

 growth from the frontals (ii), parietal (7), postfrontals (12), and mastoids (s, see Pis. 1 1, 

 and MA). 



The bony carapace is composed externally of a series of median and symmetrical 

 pieces (fig. 1, cJi, f<\—s\\, pij), and of two series of unsymmetrical pieces (phs, m\ — 12) 

 on each side. The median pieces have been regarded as lateral expansions of the 

 summits of the upper vertebral (neural) spines,! the median lateral pieces as similar 



* Reports of the British Association for the Advancement of Science, 1841, p. \CM. 

 t Cuvier, Lemons d'Anatomie Comparee, torn, i (1799), p. 212. 



B 



