mabsh.] FAMILIES OF SAUKOPODA. 165 



When first found these fossils were supposed to be from the Dakota 

 group, but their upper Jurassic age was soon after determined by the 

 writer from evidence that placed the horizon beyond dispute. The 

 name Titanosauriis montanus was given by the writer to this reptile 

 when first described, but as the generic designation proved to be preoc- 

 cupied, Atlantosaurus was substituted. 



A third Rocky Mountain locality which proved to be especially pro- 

 lific in dinosaurs was found the following year, by Mr. M. P. Felch, a 

 few miles north of Canyon, Colo., and in essentially the same horizon 

 as the last-mentioned locality. Here were found the type specimens 

 of some of the most interesting dinosaurs yet discovered in this country, 

 all of them in fine preservation, and not infrequently in the exact posi- 

 tion in which they died. Other localities of interest have been found in 

 the same region. 



Another locality of Sauropoda, more recently explored by the writer, 

 is in South Dakota, on the eastern slope of the Black Hills. This is 

 the most northern limit now knowu of the Atlantosaurus beds, which 

 form a distinct horizon along the eastern flanks of the Rocky Mountains, 

 marked at many points by the bones of gigantic dinosaurs, for nearly 

 500 miles. The strata are mainly shales or sandstones of fresh-water 

 or estuary origin. They usually rest unconformably upon the red Tri- 

 assic series, and have above them the characteristic Dakota sandstones. 



On the western slope of the Eocky Mountains the Atlantosaurus beds 

 are also well developed, especially in Wyoming, but here they have 

 immediately below them a series of marine strata, which the writer has 

 named the Baptanodon beds, from the largest reptile found in them. 

 This horizon, also of Jurassic age, is shown iu the section on page 145. 

 One of the best exposures of the entire Jurassic series may be seen near 

 Lake Como, Wyoming. 



Besides the dinosaurs, which are especially abundant, the Atlanto- 

 saurus beds also contain numerous remains of extinct crocodiles, tor- 

 toises, and fishes, and with them have been found a small pterodactyl 

 and a single bird. Many small mammals, also, have been described by 

 the writer from the same beds. 



FAMILIES OF SAUROPODA. 



The Sauropoda of the American Jurassic are worthy of special atten- 

 tion, and so f&e as now known they may be divided into four families: 

 the Atlantosauxidre, which include the largest forms; the Diplodocida3 

 and Morosauridas, both represented by gigantic species; and the small 

 Pleurocoelida?, which were apparently the last survivors of the order in 

 this country. Remains of the first three families are abundant in the 

 Atlantosaurus beds of the West, but it is not certain that all were con- 

 temporaneous. The Pleuroccelidre are especially characteristic of the 

 Potomac beds on the Atlantic coast. 



All the known members of these families were quadrupedal, with the 

 fore and hind limbs nearly equal in length. The head was very small 



