VERTEBRATE LIFE IN AMERICA. 519 



Reptiles and birds form the next great division of vertebrates, 

 the /SaurojJsida, and of these the reptiles are the older type, and may- 

 be first considered. While it may be stated with certainty that there 

 is at present no evidence of the existence of this group in American 

 rocks older than the Carboniferous, there is some doubt in regard 

 to their appearance even in this period. Various footprints which 

 strongly resemble those made by lizards, a few well-preserved re- 

 mains similar to the corresponding bones in that group, and a few 

 characteristic specimens, nearly identical with those from another order 

 of this class, are known from American Coal-Measures. These facts, 

 and some others which point in the same direction, render it probable 

 that we may soon have conclusive evidence of the presence of true 

 reptiles in this formation, and in our overlying Permian, which is essen- 

 tially a part of the same series. In the Permian rocks of Europe 

 true reptiles have been found. 



The Mesozoic period has been called the Age of Reptiles, and 

 during its continuance some of the strangest forms of reptilian life 

 made their appearance, and became extinct. Near its commencement, 

 while the Triassic shales and sandstones were being deposited, true 

 reptiles were abundant. Among the most characteristic remains dis- 

 covered are those of the genus Belodon, which is well known also in 

 the Trias of Europe. It belongs to the Thecodont division of reptiles, 

 which have teeth in distinct sockets, and its nearest affinities are with 

 the Crocodilia, of which order it may be considered the oldest known 

 representative. In the same strata in which the Belodonts occur, 

 remains of Dinosaurs are found, and it is a most interesting fact that 

 these highest of reptiles should make their appearance, even in a gen- 

 eralized form, at this stage of the earth's history. The Dinosaurs, 

 although true reptiles in all their more important characters, show 

 certain well-marked points of resemblance to existing birds of the 

 order Hatitce, a group which includes the ostriches ; and it is not im- 

 probable that they were the parent-stock from which birds originated. 



During Triassic time, the Dinosaurs attained in America an enor- 

 mous development both in variety of forms and in size. Although 

 comparatively few of their bones have as yet been discovered in the 

 rocks of this country, they have left unmistakable evidence of their 

 presence in the footprints and other impressions upon the shores of 

 the waters which they frequented. The Triassic sandstone of the 

 Connecticut Valley has long been famous for its fossil footprints, 

 especially the so-called " bird-tracks," which are generally supposed 

 to have been made by birds, the tracks of which many of them closely 

 resemble. A careful investigation, however, of nearly all the speci- 

 mens yet discovered has convinced me that there is not a particle of 

 evidence that any of these fossil impressions were made by birds. 

 Most of these three-toed tracks were certainly not made by birds ; 

 but by quadrupeds, which usually walked upon their hind-feet alone, 



