REPTILIA. 
41 
many of them are marked by a spiral line, which bears 
witness to the spiral form of the membrane in the intestine 
through which they originally passed. 
Order IX.— CHELONIA. 
The tortoises and turtles date back to the Triassic period, 
when they seem to have already assumed their most 
Fig. 39.—Carapace or dorsal shield of a small Tortoise (Hardella thurgi), 
from the Pliocene of the Siwalik Hills, India; reduced in size. The 
wavy lines show the divisions (or sutures) between the bones; the firm 
lines indicate those between the overlying horny shields, c. 1-8, costal 
bones; m. 1-11, marginal bones; n. 1-8, neural bones; nu. nuchal 
bone; py. pygal bone ; sjpy. 1,2, suprapygal shields. (After Lydekker.) 
characteristic features. Fragments of a typical Chelonian 
shell (Chelytherium obscurum) are exhibited from the Keuper 
Sandstone near Stuttgart, Germany. 
Sub-order 1 . — Trionyehia. 
The three-clawed mud-turtles appear with all their typical 
characters in the Eocene both of Europe and North America. 
Case G-. 
Wall-cases] 
18, 19. 
Cases W 
to Z. 
Wall-case • 
19. 
Wall-case 
18. 
