172 



REPTILIA. 



and the temporal fossae not roofed. The cervical vertebrae do not exhibit 

 transverse processes. There are marks of epidermal shields both on the 

 carapace and plastron. The costal bones are narrowed at the outer end, 

 so that there is a series of vacuities within the well- developed border of 

 marginal bones. The elements of the plastron also resemble those of the 

 modern Chelone. The digits bear large claws and are not much elongated ; 

 the phalangeal formula of the manus is proved to be 2, 3, 3, 3, 3. 



Pleurosternum (figs. 108, 109). This genus is known chiefly by the 

 shell, of which many fine examples have been found in the Purbeck Beds 

 of Swanage, Dorsetshire. Parts of the pectoral and pelvic arches are the 

 only other parts identified with certainty. The surface of the shell is very 

 finely pitted and deeply impressed with the suture-lines of the epidermal 

 shields. The carapace is much depressed, without vacuities, and firmly 

 united by a long bridge on each side with the plastron (fig. 108), which is 



FIG. 108. 



Pleurosternum portlandicum ; imperfect plastron, inferior aspect, showing the 

 sutures (delicate wavy lines) and impressions of the borders of the epider- 

 mal scutes or shields (bold dark lines), about one-quarter nat. size. U. 

 Jurassic (Portland Beds) ; Dorsetshire, ep.p., epiplastron ; ent.p. , ento- 

 plastron ; Tip.p., hypoplastron ; hy.p., hyoplastron ; ms.p., mesoplastron ; 

 xp. , xiphiplastron. (After Lydekker.) 



also continuous. There is a well-developed pair of mesoplastral bones 

 meeting in the middle line. A median intergular epidermal shield is dis- 

 tinctly indicated. The pectoral arch closely resembles that of the existing 

 Pleurodiran Chelys, and the coracoid is much expanded distally. The 

 pubis articulates, without sutural union, with the plastron (fig. 109). The 



