6 CHELONIA. 
The external characters are of little avail as indicative of the 
relationships of the higher groups of Chelonians ; and the characters 
on which the preceding synopsis 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. Piastral shields 11 or 12 .. Testudinida, p. 48. 
2. Plastral shields 13, an intergular being present. 
Neck retractile within the shell; no nuchal. 
Pelomeduside, p. 191. 
Neck constantly exposed; nuchal usually present. 
Chelydide, p. 206. 
B. Pectoral shields widely separated from the marginals. 
1. Tail more than half as long as the shell. 
Plastron very small, cruciform ...... Chelydride, p. 19. 
Plas hron arco ragtract re teute Vat eee Platysternide, p. 45. 
2. Tail not half as long as the shell. 
a. Digits distinct, four or five clawed. 
23 shields round the carapace and 4 or 5 on the anterior lobe of the 
PlASGrOMss Ta he Ae i AR ee Cinosternide, p. 33. 
23 shields round the carapace and 2 or 3 on the anterior lobe of the 
plastron, or 25 marginals ........ Dermatemydida, p. 27. 
b, Limbs paddle-shaped, with one or two claws. 
Chelonide, p. 180. 
II. Shell without epidermal shields. 
Limbs paddle-shaped, clawless ...... Sphargida, p. 7. 
Limbs paddle-shaped, with two claws. . Carettochelydida, p. 236. 
Digits distinct, claws three.......... Trionychide, p. 241. 
