1218 



CLASS AVES. 



Ostrich both the hallux and the second toe are wanting, and the foot con- 

 sists simply of the third and fourth digits. 



In regard to their geological distribution it should be observed 

 that the remains of Birds are generally by no means so common in 

 ossiferous deposits as those of Mammals. This scarcity is probably 

 in part due to the comparatively small size and fragile nature of the 

 bones of a large number of members of the class ; and also, as Sir 

 C. Lyell has observed, to the circumstance that " the powers of 

 flight possessed by most birds would ensure them against perishing 

 by numerous casualties to which quadrupeds are exposed during 

 floods ; " so that, " if they chance to be drowned, or to die when 

 swimming on water, it will scarcely ever happen that they will be 

 submerged so as to become preserved in sedimentary deposits, 

 since, from the lightness of the bones, the carcass would remain 

 long afloat, and would be liable to be devoured by predaceous ani- 

 mals." To these considerations must be added the absence of teeth 

 in the great majority of Birds, whereby we are deprived of evidence 

 which in the case of Mammals has thrown most important light 

 upon the nature and affinity of fossil forms. 



The earliest suggestion of the occurrence of Birds is afforded 

 by impressions of huge Sauropsidan feet (fig. 1109) found in the 



Fig. 1 109. — Sauropsidan footprint, and impressions of rain-drops ; from the Trias of the 

 Connecticut Valley. Reduced. 



reputed Triassic sandstone of the Connecticut Valley in the United 

 States. These impressions were evidently made either by Ornitho- 

 podous Dinosaurs, or by Ratite Birds ; and the occurrence of cer- 

 tain reptilian bones in the same deposits indicates that at least some 

 of them are probably of Dinosaurian origin. The absence of the 



