164 



DINOSAURS OF NORTH AMERICA. 



SAUROPODA. 



The herbivorous dinosaurs of the American Jurassic are of special 

 interest. To begin with the order Sauropoda, which includes the most 

 primitive and gigantic forms, it is an interesting fact that the. first 

 specimen found in this country was one of the rarest of the group, and 

 one of the most diminutive. A few teeth and bones only were obtained 

 by Prof. P. T. Tyson, about 1858, near Bladensburg, Md. The teeth 

 were named Astrodon by Dr. Christopher Johnston, in 1859, and in 

 1865 were described and figured by Dr. Leidy. The type specimens are 

 now in the Yale museum, and one tooth is represented below in fig. 6. 

 The strata containing these remains are known as the Potomac beds, 

 but their exact age is a matter of doubt. They have been referred by 

 some geologists to the Jurassic, and by others to the Cretaceous. 



Fin. 6. — Tooth "f Astrodon Johnstoni Leidy. Natural size. Potomac, Maryland. 

 (I, outer view, b end view, c, inner view. 



ATLANTOSAURUS BEDS. 



The first known specimen of Sauropoda from the West was secured 

 by the writer in August, 1868, near Lake Como, in Wyoming Territory. 

 This fossil, an imperfect vertebra belonging to the genus since named 

 Morosaurus, was found in the upper Jurassic clays, in the horizon now 

 known as the Atlantosaurus beds. The section on page 145 will show 

 the position of these beds in the geological scale, and their relation to 

 other deposits in which Dinosauria have been found. This locality has 

 since become one of the most famous in the entire Rocky Mountain 

 region, and the writer has secured from it remains of several hundred 

 dinosaurs, among which are many of the type specimens here described. 



Remains of an enormous dinosaurian were found in 1877, near Morri- 

 son, Colo., by Prof. Arthur Lakes and Capt. H. C. Beckwith, U. S. N., 

 and this was the beginning of a series of similar discoveries. These 

 remains, described by the writer in the American Journal of Science 

 for July of that year, proved to be those of a dinosaur far surpassing 

 in size any previously kuown, and having characters that indicated a 

 new order of these reptiles. 



