NOTE-BOOK OF A NATUEALIST. 267 



We have already seen tliat some of the freshwater-tor- 

 toises make the eggs and young of crocodilians and other 

 reptiles their prey, and the conformation of some of these 

 fossils furnishes the author of the work here cited with 

 another generalizing observation. 



After remarking that abundance of food must have 

 been produced under the influences of a climate such as 

 that which the fossil turtles enjoyed, he proceeds to the 

 inference that to some of the extinct S23ecies — which, like 

 the Chelone longicepa and Chelone 'planimentum, ex- 

 hibit a form of head well adapted for penetrating the 

 soil, or with modifications that indicate an affinity to the 

 Trionyces — was assigned the task of checking the undue 

 increase of the now extinct crocodiles and ga vials of the 

 same epoch and locality, by devouring their eggs or their 

 young, the trionyces themselves becoming, probably, in 

 return, an occasional prey to the older individuals of the 

 same carnivorous saurians. Thus did the lex talionis 

 prevail long before lawyers stained paper with their well- 

 galled ink. Thus was the balance kept up in bygone 

 ages as it now is. The same principle of mutual exter- 

 mination was, and is, and is to be ; and by this principle, 

 which to the uiiinitiated must wear somewhat of an 

 Acherontic aspect, the greatest quantity of general hap- 

 piness is secured in what would otlierwise be an over- 

 crowded world : but vce victis. 



The well-arched, thick-walled, waggon-proof, portable 

 castle, assigned by the distributive justice of Nature to 

 the larger slow land-tortoises, and those, consequently, 

 more exposed to obs.ervation and attacks, is in the turtles 

 modified to suit the element in which they principally 

 live. The carapace forming the roof is less highly arched, 

 and both it and the floor or j)lastron are lighter and less 

 completely ossified; but as the head cannot be drawn 

 back under the carapace, as in the land-tortoises, it is 

 fortified by an additional bony helmet. 



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