324 PALEONTOLOGY. 



Sir C. Lyell lias well remarked, that " the powers of flight 

 possessed by most birds would insure them against perishing 

 by numerous casualties to which quadrupeds are exposed 

 during floods." The same writer further argues, 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."* It is true 

 that the carcase of a floating bird may not sink where it has 

 died, but be carried far along the stream : ultimately, however, 

 if not devoured, its bones will subside when the soft parts 

 have rotted ; and both the compactness of the osseous tissue, 

 and the facts made known by the ornitholites of the green- 

 sand near Cambridge, of the London clay at Sheppy, and of 

 the Montmartre eocene quarry-stone, shew that they can be 

 preserved in the fossil state. The length of time during which 

 the carcase of a bird may float, doubtless exposes it the more 

 to be devoured, and so tends to make more scarce the fossil 

 remains of birds in sedimentary strata. 



Certain it is that the major part of the remains of extinct 

 birds that have as yet been found are those of birds that were de- 

 prived of the power of flight, and were organized to live on land. 



The existence of birds at the triassic period in geology, or at 

 the time of the formation of sandstones which are certainly in- 

 termediate between the lias and the coal, is indicated by abun- 

 dant evidences of footprints impressed upon those sandstones 

 which extend through a great part of the valley of the Connecti- 

 cut Eiver, in Connecticut and Massachusetts, North America. 



The footprints of birds are peculiar, and more readily dis- 

 tinguishable than those of most other animals. Birds tread 

 on the toes only ; these are articulated to a single metatarsal 

 bone at right angles to it, and they diverge more from each 

 other than in other animals. 



Not more than three toes are directed forward ;t the fourth, 



* Principles of Geology, ed. 1847, p. 721. f Save in the Swift. 



