6 CHELONIA. 



The external characters arc of little avail as indicative of the 

 relationships of the higher groups of Chelonians : and the characters 

 on which the i)rcccdiiig synoiisis is founded are mostly osteological. 

 The following key is appended for the use of those not fully expe- 

 rienced in the "physiognomy" of Chelonians, who have not an 

 opportunity of investigating the osteological characters ; it is made 

 up of readily ascertainable peculiarities, however trivial, and with 

 its aid the family to which every known recent Chelonian belongs 

 ought to be recognized without hesitation. 



I. Shell covered with epidermal, horny shields. 



A. Pectoral shields of plastron in contact with the marginals. 



1. riastral shields 11 or 12 . . TestudinidsB, p. 48. 



2. Plastral shields 13, an intergular being present. 



Neck retractile within the shell ; no nuchal. 



Peloi edusidsB, p. 101. 



Neck constantly exposed: nuchal usually pre; .'nt. 



Chelydidse, p. 200. 



B. Pectoral shields widely separated from the marginals. 



1. Tail more than half as long as the shell. 



Plastron very small, cruciform Chelydi'idse, p. 19. 



Plastron large Platysternidae, p. 45. 



2. Tail not half as long as the sheD. 



a. Digits distinct, four or five clawed. 



23 shields round the carapace and 4 or 5 on the anterior lobe of the 

 plastron Cinosternidse, p. 33. 



23 shields round the carapace and 2 or 3 on the anterior lobe of the 

 plastron, or 25 marginals DermatemydidaB, p. 27. 



h. Limbs paddle-shaped, with one or two claws. 



Chelonidae, p. 180. 



II. Shell without epidermal shields. 



Limbs paddle-shaped, clawless Sphargidffi, p. 7. 



Limbs paddle-shaped, with two claws. . CarettochelydidsB, p. 23(5. 

 Digits distinct, claws three Trionychidte, p. 241. 



