174 DR. J. E. GRAY ON THK BOX TORTOISES. [May 12, 



to study them properly, do theorists wish to support the theory that 

 species gradually pass into each other, and have been derived, or 

 rather have originated, from such transformations. Never was a 

 theory more baseless, as far as our knowledge is concerned. 



This imperfection of our knowledge is specially the case with re- 

 spect to exotic Tortoises, where we sometimes only procure the shell, 

 at other times the animal with the shell in a more or less perfect 

 condition ; and when the latter is procured, we find that the conclu- 

 sions that we had come to as regards the probable form of the animal, 

 or some part of it, are more or less incorrect, and we are thus obliged 

 to reconsider the situation the species occupies in the series. 



Having lately received some more perfect specimens of some of 

 the Indian Box Tortoises, I am induced to suggest their arrangement 

 as follows : — 



The Tortoises belong to the tribe Cistudina, are characterized by 

 having the sternum attached to the back by a ligamentous suture 

 on each side, and being divided across the centre by a similar cross 

 suture, leaving the front and hind lobe more or less moveable. 



In the normal Cistudince, which have the lobes of the sternum 

 moveable at all ages, the cartilaginous sutures and the suture between 

 the pectoral and ventral shields of the sternum are at the same situ- 

 ation ; and the lobes of the sternum are broad, as broad as the open- 

 ing of the thorax, and cover the legs when they are contracted. 



The normal Cistudince may be divided into genera, according to 

 the more or less aquatic habits of the animal, as indicated by the 

 structure of the feet. 



I. Sternum-lobes unequal ; front shorter, almost free from the sym- 

 physis. The hind foot slender, elongate ; toes very unequal, 

 second longest. N. America. 



1. CiSTUDO. 



Thorax convex, solid ; sternum rounded or truncated before and 

 behind ; the front lobe smaller, almost free from the symphysis. 

 The fore legs with large shields in front ; the toes short, enclosed, 

 not webbed, with short conical claws. The hind feet elongate, nar- 

 row, with the second toes produced ; the rest short, nearly enclosed, 

 not webbed ; the soles of the feet with subequal moderate-sized 

 scales, the hinder edge rounded. 



N. America. 



* The hind feet with small hinder or outer fourth toes. Cistudo. 



Ctstudo CAROLINA, Gray, Cat. Shield Rept. B.M. p. 39. 



Of which C. ornata and C. major, Agassiz, seem to be varieties. 



** The hind feet without any small fourth toes. Onychotria. 

 Cistudo mexicana, Gray, Cat. I. c. p. 40. 



See also C. triunguis, Agassiz, which is said to be smaller than 

 C. Carolina and C. mexicana. 



I 



