164 Dr. J. E. Gray on the Development and Cohesion 



of the hinder pair of bones, which are much larger than usual, 

 and united by a straight median suture ; it is covered by the 

 preanal and anal plates. 



This form is described from the genera Swanlca and Kino- 

 sternon, where this modification of the sternum is in its most 

 developed state. The sternum of the other genera is often 

 narrow, and a portion is only slightly mobile, and the plates 

 (which cover it) are diminished in number or coherent to- 

 gether. 



The land-tortoise Pyxis and the fluviatile Sternothcerus have 

 the front lobe of the sternum free ; but it is only the front 

 lobe of the sternum that is free, the abdominal portion being 

 firmly united to the marginal portion of the back, as in the 

 tortoises with an undivided sternum. 



The true Box Tortoises consist of the family Cistudinidse. 



The Trap Tortoises consist of the Chelydradaj. 



3rd. All the other families of Freshwater Tortoises or Ter- 

 rapins have a simple undivided sternum. 



One might premise that these three forms would each have 

 a distinct development of the bones of which the sternum is 

 formed, or that the bones of each of the three forms would be of 

 the same shape and developed in the same manner during the 

 growth of the animal. But the examination of the young 

 specimens which have come under my observation (which, un- 

 fortunately, are too few for the proper study of the subject) proves 

 this not to be the case ; and I am inclined to think that the 

 study of the development of these bones may be subservient to 

 the natural arrangement of these animals, and also a great 

 assistance in tlie determination of the fossil species. 



The development of bones of the sternum of Freshwater 

 Tortoises may be divided into three series, thus : — 



I. The nine bones in the very young state are well developed, 

 the lateral bones being largely developed and covering the 

 greater part of the middle of the sternum. There is a mode- 

 rate-sized vacant space in the middle of the sternum, and a 

 smaller one at the hinder part of the sternum, between the 

 inner hinder angles of the lateral pair of bones and the inner 

 side of the hinder plates, and an oval space on each side of 

 the angular odd bone between it and the inner front edge of 

 the anterior lateral bone. 



This form is well exhibited in the sternum of Malaclemmys 

 concentrica (Plate V. fig. 1) of the family Malaclemmydse, and of 

 Pelomedusa subrufa (fig. 2), family Hydraspidas. The anterior 

 lateral bones are larger and more developed in Malaclemmys 

 than in Pelomedusa] and this appears to be the most usual form 

 of the sternum of the Freshwater Tortoises. 



