XXIV 
INTRODUCTION. 
muddy bottoms of rivers; and the neck is as extensile and as capable of being 
rapidly protruded as in the Chelydra. Colonel Sykes informs me that in 
several specimens of the Trionychidce taken from the Ganges, which he opened, 
the stomachs were found filled with Uniones, the shells of which had been 
broken, previously to their being swallowed, iftto angular fragments which still 
adhered to the mantle, the bodies of the mollusca being still whole. This 
would appear to indicate that they seek their prey principally amongst the 
mud at the bottom of rivers, a situation in which their for m enables them so 
readily and so completely to conceal themselves. 
From the carnivorous fresh-water tortoises which have just been mentioned 
we advance at once and without the intervention of any genus of interme¬ 
diate habits, to the Cheloniadce , which, as far as we yet know, are generally 
though not exclusively vegetable feeders. We have seen in the passage from 
the land to the fresh-water groups, two distinct and well-marked gradations 
towards the animal aliment of the typical fluviatile form; of which the ha¬ 
bits of Terrapene clausa constitute the first step, and a further advance is seen 
in the European species of the same genus. But in the transition from the 
fluviatile to the marine forms, no connecting link, as far as regards food, has 
hitherto been discovered. In the structure, at least, of the external covering, 
the Coriaceous Turtle ( Sphargis mercurialis), following the Trionychidce , may 
indicate a passage from the fresh-water to the marine group, and there is a 
possibility that in this species we may also ascertain those omnivorous habits, 
which would fill that hiatus in the nature of their food, which, according to 
our present information, still exists between the families in question—for I 
have hitherto failed in finding any account of the food of the Coriaceous Turtle 
in any author. The other marine turtles feed very principally on marine and 
maritime plants, especially the Turtle Grass as it is commonly called. On the 
shores of those oceanic islands in which these animals breed, and where they 
are found in the greatest numbers, this vegetable constitutes their principal 
nourishment. It is however also stated upon good authority, that the Logger- 
head Turtle especially ( Chelonia Carettd) feeds also on molluscous animals: 
and in default of their favourite vegetable food, there appears no doubt that all 
the marine turtles may be sustained with animal nourishment. The stories 
