THE FOSSIL FOOTMARKS OF THE UNITED STATES. 137 



tolerable success ; so true is the correlation between different parts 

 of animals. Hitherto, as I shall endeavour to show in this paper, 

 only a small part of the characters that have a permanent value in 

 distinguishing the feet have been pointed out, merely because they 

 are not needed for living animals. Nevertheless, where only a 

 mould or cast of the foot remains, they may be of great service. 



I might add, in this connection, that the classes of animals which 

 seem to have made the fossil footmarks are of all others most 

 easily distinguished by their feet ; I mean reptiles and birds. 

 The chief difficulty in the case lies in the fact, that, in the red-sand- 

 stone period, some of these animals seem to have differed not a 

 little in their structure from the tribes now living. The sure 

 laws of comparative anatomy, however, are not violated. 



In the fifth place, many fossil animals have been described from 

 characters no more numerous, or definite, than those derived from 

 their feet alone. A single bone or the fragment of a bone is, 

 indeed, sometimes alone sufficient to enable the comparative anato- 

 mist to construct the whole animal. But it is not every bone that 

 will do this ; and as to plants, it is still more difficult to make out 

 their true place in the botanical scale from single parts. And we 

 know that, in many instances, animals have been named and de- 

 scribed which were subsequently found to have been referred even 

 to the wrong class ; as, for example, the Pterodactyle and Zeuglo- 

 don. Indeed, the possession of an entire skeleton is not always 

 sufficient to distinguish the species, nor even the genus (Ossemens 

 Fossiles, Tom. III. p. 524, 3d ed.). Fossilization usually obscures 

 the characters of organic beings ; and every possible degree of un- 

 certainty may be found in the catalogues of fossil animals. Yet in 

 all cases, except the one under consideration, the princijile seems to 

 have been acted on, to give a name to an unknown animal, exhum- 



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