166 DR. J. E. GRAY ON THE TORTOISES. [Mar. 11, 
almost entir ely covered with a bony case formed of the dilated zygo- 
matic arch, as in the Sea-Turtles (Cheloniade), and the other with 
those muscles only covered with skin, and protected externally by a 
broad band-like zygomatic arch, as in the Tortoises (Testudinide 
and Trionychide); and I observed that the same difference in the 
form of the skull was to be observed in the genera of the Emydide ; 
but some genera, as Geoemyda and Cistudo, like several genera of 
Chelydide, are even without any zygomatic arch, the ‘temporal 
muscles being only covered with skin between the orbit and the 
tympanic bone as on the temple and crown. 
The families may be divided, according to the state of the temple, 
thus :— 
Temple bony. Temple fleshy. 
Ie pFect clavate, (ernestniel vey ce acs oeey asks si Mee eee. oeaar Testudinide. 
1]. Feet palmate. Fluviatile. 
A. Thorax covered with bony plates. 
a. Pelvis free below ; sternal shields 
BLOF U2 rss. seas pemeanes ae gees Platysternide. Cistudinide. 
Emydide. 
Chelydrade. 
b. Pelvis attached to the sternum ; 
sternal shields 13 ............... Podocephalide. Chelydide. 
bse Piorax(COVErea with skims 1)2.-s0- a8 Meese otenerees eo ss Trionuchide. 
WD: Reetiintshapedys:. tac. cntee ce ecisnarscresses Cheloniada. 
Sphargidide. 
The horny beak of these animals not only forms a cutting instru- 
ment for the separation of the food from the mass, but it also covers 
the chewing surface on the sides of the jaws, there being a more or 
less extended plate on the inside of the jaws for this purpose. In 
some the surface of the bone and the horny covering is smooth, as in 
Malaclemys and Chelydra. In general there are one or more ridges 
on the upper jaw fitting into grooves in the lower jaw. In the Tor- 
toises and some of the more terrestrial Hmydide, the ridge and groove 
are simple; in the more aquatic Terrapims (as Pseudemys and Ba- 
tagur) they are more numerous and wider. Unfortunately, the 
form of the masticating surface is not to be usually seen in stuffed 
specimens; so that it is only known in a limited number of species. 
It must have great influence, or, rather, it shows that there is great 
variation in the habits of the animals, and ought to be studied for 
the natural arrangement of the groups. Indeed I can only regard 
the notes I am now making asthe breaking of the sod, and consider 
that much has to be done before one can arrive at a satisfactory 
history of the habits and structure of these creatures, and form an 
arrangement of them consistent with their habits and manners and 
peculiarities. 
I. LAND-TORTOISES—TESTUDINIDZ. 
Skull solid. Orbit complete, lateral, large, hinder edge moderate. 
Zygomatie arch strong, well developed, united to the ear-bone be- 
hind, with a large cavity for the temporal muscle above. Temporal 
muscles covered with skin or horny plates. 
