MONOGRAPH 



ON THE 



FOSSIL REPTILIA OF THE LONDON CLAY. 



Order.— CHEL ONI A. 



Family — Marina. 

 Genus — Chelone. 



The majority of the Fossil Chelonians of the Eocene tertiary deposits, defined or 

 described in my ' Report on British Fossil Reptiles,' belonged to the marine division 

 of the order, and to the genus Chelone; and as the species of this genus 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 those descriptions of the Chelonian reptiles which fall to my 

 share of the present Monograph. 



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 mi/das 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 (n), parietal (7), postfrontals (12), and mastoids (s, see T. I, 

 T. Ill, T. XV). 



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

 pieces (fig. 1, ch, si — s\ \,py), and of two series of unsymmetrical pieces (ph—8, mi — 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 



* Cuvier, Lecons d'Anatomie Coniparee, torn, i (1799), p. 212. 



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