316 PALEONTOLOGY. 



roof are least complete in the marine turtles ; the ribs extend 

 beyond the costal plates leaving un ossified intervals, and their 

 ends penetrate cavities in some of the marginal plates, like 

 teeth in sockets (fig. 108, a). The other elements of the cara- 

 pace are united together by marginal sutures. Analogous 

 vacuities are left in the plastron (fig. 108, b). In terrapenes 

 and tortoises ossification obliterates such vacuities ; the costal 

 plates unite by sutures to the marginal ones, and the hyo- and 

 hypo-sternals not only unite along the mid-line of the plastron, 

 but are joined laterally by sutures with more or less of the 

 marginal plates, forming the side-walls of the bony chamber. 

 In all chelonia, save the mud-turtles (Trionycidce), the exterior 

 of the elements of both carapace and plastron, are impressed 

 by the horny scutes which coated the bony elements, whereby 

 the shapes and proportions of such scutes can be recognized 

 after they have perished. The marginal impressions of the 

 five medial or "vertebral" scutes are shewn in the carapace 

 (fig. 1 08, a), at v 1 to v 5 ; the lines going out from the lateral 

 angles of these, mark the boundaries of the " costal " scutes. 

 In the fossil plastron (fig. 109), the line between hu andjje, 

 defined the "humeral'"' and "pectoral" scutes; that between 

 pe and ad defined the "pectoral" and "abdominal" scutes; 

 that between fe and an, defined the "femoral" and "anal" 

 scutes, etc.* 



The turtles, being unable to withdraw the head within 

 their shell, have it large and well ossified, as in fig. 91, B, 

 where the postfrontal, g, the mastoid, m, and squamosal, h, 

 form a continuous bony vault over the true cranium ; but in 

 the land, and most fresh-water, tortoises, the temporal fossae 

 are exposed. The bones of the limbs are modified according to 

 the medium of life and locomotion ; but in all they are solid. 



* Applications of the above analysis and nomenclature of the parts of the 

 carapace and plastron to the reconstruction of fossil Chelonia, will be found in 

 the Author's "History of British Fossil Reptiles," 4to, parts 1-3. 



