of the Bones of the Sternum of Chelomans. 169 



and at length irregularly semicircular, rather longer than broad, 

 nearly close together, and oblique to each other. 



The other species [E. ceylonensis), when adult, has the hinder 

 pair of callosities subquadrangular, parallel, and nearly united 

 by a straight inner edge and a large rounded anterior callosity. 



Sea- Turtles. 



The number of the sternal bones of Turtles is the same, and 

 the first pair and the odd bone on the inside of them are of the 

 same form, as in the Terrapins ; but they always remain more 

 or less separate from one another, and do not enlarge, solidify, 

 and consolidate into a continuous bony disk. 



As in the Terrapins, the bones of the sternum in tlie young 

 Turtles are found in two forms. In the true Turtles ( Chelonia) 

 (PL VI. fig. 4) the three hinder pairs of lateral bones are 

 always expanded and furnished with radiating lobes on the 

 inner and outer edges. These lobes are very uniform in 

 their direction and generally in their form, and afford very good 

 characters for the distinction of the species and their division 

 into groups. In the Luth {Sphargis) (PL VI. fig. 5) the 

 sternal bones in the young state are very narrow, cylindrical 

 and weak, merely forming a slight framework to the circum- 

 ference of the sternum, and the two front pairs form a group 

 which is separated by a considerable space on the side of the 

 sternum from the part of the ring formed of the two hinder 

 ateral pairs, being in this respect somewhat like the sternum 

 of the young Land-Tortoises, but consisting of slight cylin- 

 drical rudimentary bones instead of the broad expanded ones 

 of that group. 



The study of the development of the sternum of the tor- 

 toises has brought out affinities between groups that have not 

 hitherto been observed ; and no doubt, as the state of the bones 

 in more young specimens is known, it will greatly add to our 

 knowledge of the relations which the genera bear to each other. 

 This may be exhibited by the following table, which will lead 

 the zoologist and comparative anatomist to consider this sub- 

 ject, and see many affinities between groups that have hitherto 

 been considered very different, and divergences in groups that 

 have hitherto been regarded as allied. 



Chelonians may be divided thus : — 



I. The bones of the sternum, and also of the dorsal disk and 

 margin, of the adult animal all united together and con- 

 solidated as if they were a single bone. 



a. The bones of the sternum in the young animal expanded, 



