378 FAUNA ANTIQUA SIVALENSIS. 



IV. — Anatomical Description of the Colossochelys 

 Atlas, beingMS. Notes for the Lecture delivered 

 AT THE British Association in September 1844. 



The Sternum. — In the Chelonians the sternum is a highly 

 developed apparatus, and consists in all the species invari- 

 ably of 9 pieces, 8 of which are pairs and 1 an odd piece ; 

 2 epistemals, 2 hyostemals, 2 hypostemals, 2 xiphosternals, 

 and 1 entosternal. In the Chelonia and Trionyx the pieces 

 are detached ; in the Land Tortoises and Emydce they are 

 solid. The fragments of the Colossochelys show that the 

 episternals were of an amorphous thick form, cleft at the 

 apex, with a deep cuneiform keel underneath. The ento- 

 sternal shows an exact correspondence in form and ha every 

 particular with several Testudines. In the specimen in the 

 Zoological Society the length of sternum is 32 inches, and 

 the length of epi- and ento-sternal pieces 10 inches. In Colos- 

 sochelys, the epi- and ento-sternal j)ieces measure 35 inches, 

 and the inferred length, on the same proportions, of the entire 

 plastron is 9 feet 4 inches. The lateral curved expansions 

 are also to be noted. All these concurrent marks are found in 

 no other Chelonians than the Testudines. Hence the first in- 

 dication of its being a Tortoise is exhibited by the sternum. 



The xiphosternals are cleft behind under the tail. In 

 the fossil, in thickening, marginal ridge, and general charac- 

 ters they resemble the living Tortoise. In the hyposternals 

 there is a similar correspondence and thickenmg. The whole 

 of the sternum indicates a testudinate animal of enormous 

 size, and a true Land-Tortoise. 



Character of the Cara2m.ce. — The dorsal shield consists of 10 

 pairs of ribs in 8 sutured pieces (the 1st and 10th behig con- 

 fluent with the 2nd and 8th), connected with as many 

 vertebral plates ; there are 11 vertebral plates in all (the last 

 or supra-caudal having no attachment to the ribs), and 11 mar- 

 ginal or sterno-costal pieces correspoudmg to the cartilages of 

 the ribs. There are 10 dorsal vertebrae, the fia'st free and 

 homomorphous with the cervical vertebrae, of which there are 

 7 (apparently 8). 



The carapace of the Land Tortoises is short, very elevated 

 and arched, and more or less vertical at the sides, as com- 

 pared with other Chelonians. The thickness of bone in 

 the convexity is almost m an inverse ratio to the size. The 

 physiological reason of this is that the smaller the animal the 

 more liable it is to injury, and it requires a greater arch to 

 sustain it. There are no complete pieces in the fossil 

 remains, and the reasons of this are the curvature and thinness. 

 The most marked pieces in the carapace are the sterno-costal, 



