FOSSIL CHELONIAN REPTILES OF THE 



Genus — Pleurosternon. 



Char. Gen. — Testa depressa lata, complanata ; sternum integrum, ossihus undecim 

 composittim, per ossicuKs marginalihus cum testa conjunctum ; scutis suhmarginalihus inter 

 scuta axillarid et inguinalid positis. 



As a general rule the vertebrate animals of the secondary 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 exemplified in the 

 present genus of Chelonian 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, Tab. Ill) and 

 " hyposternals" {pg, 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 hsemapophyses, 

 — 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, proposed for the remarkable extinct Chelonia about to 

 be described, it is desired to intimate the characteristic furnished by the additional 

 number of inferior rib-elements (hsemapophyses, or "cartilagines costarum" of Anthro- 

 potomy) composing the under-shell or plastron, which part naturalists, influenced by 

 the views of Geoffrey St. Hilaire, have usually described under the arbitrary name 

 of " sternum." 



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, afforded by 

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

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

 the carapace and plastron, forbid its being referred to \h% fluvial tribe, represented by 

 the Trionyces ; the depressed shape of the carapace excludes it from the terrestrial 

 tribe of true Tortoises ; and we arrive, therefore, by the way of exclusion, to the asso- 

 ciation of the genus in c^uestion with the Terrapenes and other members of the family 

 Paludinosa of the eminent Erpetologists, MM. Dumeril and Bibron. 



