WEALDEN CHELONIANS. 179 



SUPPLEMENT TO VOL. I.— CHAPTER I. Order -CHELONIA. 



Genus — Pleueosternon, Oiven. 



As a general rule the vertebrate animals of the Mesozoic strata manifest, in the 

 modifications of their structure, a nearer approach to the archetype of their sub- 

 kingdom than the tertiary and existing Vertebrates do. This rule is exempHfied 

 in the present genus of Ohelonian Reptiles by the accessory osseous pieces that 

 enter into the formation of the plastron, and which are interposed, as an additional 

 pair of bones, between those more constant parial elements called " hyosternals " 

 {hs, Plate 54) and " hyposternals " (p s, ib.), and which alone articulate with the 

 marginal pieces (m, m) in existing Emydians. At least, if we adopt the general 

 homology of the parial elements of the plastron, indicated by the development of 

 that part, viz. as being hasmapophyses, — an increased number of such pieces, 

 making them to that degree more equal in number with the pleurapophyses of the 

 carapace, offers an obvious tendency to a return to the normal type ; and the fact 

 of a genus or family of extinct secondary Chelonians manifesting such increase in 

 the number of parial pieces, gives additional support to the conclusions as to the 

 nature of the plastron arrived at from a study of that part in the embryos of 

 existing species. 



By the name Pleurosternon it is desired to intimate the characteristic furnished 

 by the additional number of inferior rib-elements (hjemapophyses, or " cartilagines 

 costarum " of Anthropotomy) composing the under-shell or plastron. 



The extent of the ossification of the carapace and plastron, and the firm union 

 of the roof and floor of the bony chamber by the medium of the side-walls, afibrded 

 by certain marginal plates, prove the genus not to belong to the marine Ghelonia ; 

 the presence of the marginal plates, and the impressions of the horny scutes which 

 covered the carapace and plastron, forbid its being referred to the fluvial tribe, 

 represented by the Trionyces ; the depressed shape of the cai^apace excludes it from 

 the terrestrial tribe of true Tortoises ; and we arrive, therefore, by the way of 

 exclusion, to the association of the genus in question with the Terrapenes and 

 other members of the family Paludinosa. 



Pleurosternon concinnum, Owen. Plates 53, 54, 



The subjects of the above Plates consist of a nearly entire carapace and 

 plastron. They are from the Purbeck Limestone,, Swanage, Dorsetshit-e. 



