5 2o THE POPULAR SCIENCE MONTHLY. 



and only occasionally put to the ground theh' smaller anterior extremi- 

 ties. I have myself detected the impressions of these anterior limbs in 

 connection with the posterior footprints of nearly all of the supposed 

 "bird-tracks" described, and have little doubt that they will event- 

 ually be found with all. These double impressions are precisely the 

 kind which Dinosaurian reptiles would make, and, as the only char- 

 acteristic bones yet found in the same rocks belong to animals of this 

 group, it is but fair to attribute all these footprints to Dinosaurs, 

 even where no impressions of fore-feet have been detected, until some 

 evidence appears that they were made by birds. I have no doubt that 

 birds existed at this time, although at present the proof is wanting. 



The principal genera of Triassic reptiles known from osseous re- 

 mains in this country are Arnphisaurus (Megadactylus), from the 

 Connecticut Valley ; Bathygnathus, from Prince Edward's Island ; 

 Belodon, and Clepsysaurus. Other generic names which have been 

 applied to footprints and to fragmentary remains need not here be 

 enumerated. A few remains of reptiles have been found in undoubted 

 Jurassic rocks of America, but they are not sufficiently well deter- 

 mined to be of service in this connection. 1 Others have been reported 

 from supposed Jurassic strata, which are now known to be Cretaceous. 

 It will thus be seen that, although reptilian life was especially abun- 

 dant during the Triassic and Jurassic periods, but few bones have 

 been found. This is owing in part to the character of most of the 

 rocks then formed, which were not well fitted for preserving such 

 remains, although admirably adapted to retain footprints. 



During the Cretaceous period, reptilian life in America attained 

 its greatest development, and the sediments laid down in the open 

 seas and estuaries were usually most favorable for the preservation 

 of a faithful record of its various phases. Without such a perfect 

 matrix as some of these deposits afford, many of the most interesting 

 vertebrates recently brought to light from this formation would prob- 

 ably have remained unknown. The vast extent of these beds insures, 

 moreover, many future discoveries of interest. 



In the lowest Cretaceous strata of the Rocky Mountain region, 

 the Dakota group, part of which at least represents the Wealden of 

 Europe, remains of Chelonia, or Turtles, Crocodiles, and Dinosaurs 

 occur, the last being especially abundant. The Chelonia; although 

 known from the Jurassic of Europe, here appear for the first time in 

 American rocks. Some of the earliest forms are allied to the modern 

 genus Trionyx. In the higher Cretaceous beds, some Chelonians of 

 enormous size have been found. They belong to the genus Atlan- 

 tochelys, which has the ribs separate, as in the existing Sphargis,* 



1 Since this address was delivered, I have determined the beds containing gigantic 

 Dinosaurs, on the flanks of the Rocky Mountains, to be upper Jurassic, and called them 

 Atlantosaurus Beds. (See frontispiece, section.) These strata were formerly referred 

 to the Dakota group, or base of the Cretaceous. 0. C. M. 



2 Sphargis, a genus of turtles inhabiting the Atlantic and Mediterranean. They are 



