CROCODILEANS. 251 
fishes, we might, a priori, expect that if any 
Crocodilean forms had then existed they would 
most nearly have resembled the modern Gavial. 
And we have hitherto found only those genera 
Avhich have elongated beaks, in formations an- 
terior to, and including the chalk; whilst true 
Crocodiles, with a short and broad snout, like 
that of the Cayman and the Alligator, appear 
for the first time in strata of the tertiary periods, 
in which the remains of mammalia abound.* 
During these grand periods of lacustrine 
mammalia, in which but few of the present 
genera of terrestrial carnivora had been called 
into existence, the important office of controlling 
the excessive increase of the aquatic herbivora 
appears to have been consigned to the Croco- 
diles, whose habits fitted them, in a peculiar 
degree, for such a service. Thus, the past his- 
tory of the Crocodilean tribe presents another 
example of the well regulated workings of a 
* One of these, found by Mr. Spencer in the London clay of 
the Isle of Sheppy, is engraved, PI. 25', Fig. 1. Crocodiles of 
this kind have been found in the chalk of Meudon, in the 
plastic clay of Auteuil, in the London clay, in the gypsum of 
Mont Martre, and in the lignites of Provence. 
The modern broad-nosed Crocodileans, though they have the 
power to capture mammalia, are not limited to this kind of prey; 
they feed largely also on fishes, and occasionally on birds. This 
omnivorous character of the existing Crocodilean family, seems 
adapted to the present general diffusion of more varied kinds of 
food, than existed when the only form of the beak in this family 
was fitted, like that of the Gavial, to feed chiefly on fishes. 
