THE DIVISIONS OF THE CHELONIA. 179 



centrale. In Clielone^ Emys^ Testudo^ and Trionyx the centraU 

 is united with the astragalus y and in Emys^ the calcaneuyn 

 coalesces into the astragalus^ so that the proximal portion of 

 the tarsus consists of one bone. In the distal series the two 

 fibular tarsals are united into a cuboid bone. There are five 

 digits, and the fifth metatarsal has a peculiar form, as if bent 

 upon itself at right angles, in the middle of its length. 



In the Testudinea there are only two phalanges in each 

 digit of the pes. 



The Chelonia are divisible into the Testudinea, the Emy- 

 dea, the Trionychoidea, and the Euereta. 



1. The Testudinea have the horny jaws naked and cutting, 

 or denticulated. The eyes are lateral, the tympanic mem- 

 brane is exposed, the short and thick limbs have the toes 

 (all of which have nails) bound together by the integument. 

 The horny plates of the carapace and plastron are well de- 

 veloped. 



The Land Tortoises belong to this division. The carapace 

 is usually very convex, and sometimes (as in the genus Pyxis) 

 the anterior part of the plastron is movable, and can be shut 

 up like a lid. In Clnyxis, the hinder part of the carapace is 

 similarl}'- mobile. 



2. The Emydea have, usually, horny cutting jaws, un- 

 covered by lips ; the tympanum exposed, and the limbs more 

 slender than in the Testudinea^ with five-clawed digits, which 

 are only united by a web. The horny plates of the carapace 

 and plastron are well developed. 



These are the River and Marsh Tortoises. They are further 

 divisible into two groups, in the one of Avhich, the Terraioenes, 

 the pelvis is free, the neck bends in a vertical plane, and the 

 head is almost completely hidden by the carapace when re- 

 tracted {Emys^ Cistudo^ Chelydra). In Gistudo^Ginostenum, 

 and Staurotypus, the hinder part of the plastron is mobile. 

 In the other division, the Gkelodines^ the pelvis is fixed to the 

 carapace and plastron, the neck bends sideways, and the 

 head cannot be completely retracted under the carapace 

 ( Ghelys^ GJielodina.) 



3. In the TrionycJioidea (Mud or Soft Tortoises), the jaws 

 have an external cutaneous lip ; the nasal organ is prolonged 

 into a kind of snout, and the head is covered by a soft skin 

 without any visible tympanic membrane. The limbs are flat- 

 tened, somewhat finlike, and pentadactyle ; but only three 

 iigits have nails. The integument develops no horny plates, 



