1092 



CLASS REPTILIA. 



lowing sections, and may probably be regarded as the survivors of 

 the earlier ancestral types from which those two sections took origin. 

 They are all characterised by the presence of a mesoplastral bone, 

 and of an intergular shield in the plastron ; and the pelvis may or 

 may not be connected with the xiphiplastrals. The entoplastral is 

 rhomboidal. The skull and cervical vertebrae are unknown. 



Family Pleurosternid^e. — All the members of this section may 

 be, at least provisionally, included in this family. In addition to 

 the characters given above, it may be observed that the shell is 

 fully ossified, and that the carapace has a complete series of 

 neural bones, of which the hindmost articulates with the anterior 



suprapygal bone. On the sup- 

 position that the Chelonian 

 plastron is derived from a 

 system of abdominal ribs like 

 those of the Rhynchocephalia 

 and Sauropterygia, it will be 

 evident that the mesoplastral 

 of the present group is an 

 archaic feature. 



In the typical genus Pleuro- 

 sternum {Megasternum or Di- 

 gerrhum), which occurs com- 

 monly in the English Purbeck, 

 and is also found in the Port- 

 land Oolite, the shell is broad 

 and depressed, with complete 

 mesoplastrals (fig. 1012), a 

 large and wide entoplastral, a 

 single intergular shield, and no 

 nuchal shield. In the adult 

 the pubis articulates with a 

 smooth facet on the xiphi- 

 plastral, thus foreshadowing the 

 complete sutural union which 

 occurs between these bones in 

 the Pleurodira ; but in the young it appears that there was no such 

 articulation. The neural bones of the carapace are hexagonal and 

 comparatively long ; while the vertebral shields (as in so many of 

 the earlier Chelonians) were relatively wide. Further, inframarginal 

 shields (shown in fig. 1012) were developed between the shields of 

 the plastron and the marginal shields of the carapace ; while the 

 extremities of the xiphiplastrals were notched. The bones of the 

 pectoral girdle and the humerus approximate to those of the existing 

 Pleurodiran genus Chelys. 



Fig. 1012. — The plastron of Pleurosternum 

 Bullocki; from the Purbeck of Dorsetshire. One- 

 third natural size, ig, Intergular scute ; g, Gular 

 do. ; pec, Pectoral do. ; ab, Abdominal do. ; fern, 

 Femoral do. ; an, Anal do. ; e.p, Epiplastral 

 bone ; entp, Entoplastral do. ; hy.p, Hyoplastral 

 do. ; ms.p, Mesoplastral do. ; hp.p, Hypoplastral 

 do. ; xp, Xiphiplastral do. 



