90 



0. C. Marsh — New Genus of Sauropoda. 



The most abundant fossils obtained are remains of Dinosaurian 

 reptiles, three orders of which are represented, and in the pres- 

 ent article, some of the new forms are described. Associated 

 with these are remains of crocodiles and tortoises, also of 

 Jurassic types, some fishes, and a few mollusks. A number 

 of plants have been found, mainly conifers and cycads. The 

 strata containing these fossils are evidently of lacustrine origin. 



Pleurocoelus nanus, gen. et sp. nov. 



The most common fossils secured thus far from the Potomac 

 formation are the remains of a small Dinosaur which clearly 

 belongs to the Sauropoda, but is by far the most diminutive 

 member of this group yet discovered. Portions of the skull, 

 vertebrae, and limb bones of several individuals have been 

 obtained, and these agree so nearly that they may be referred 

 to one species. They differ somewhat in size, owing appar- 

 ently to a difference in age. 



In comparing these remains with the Sauropoda now known, 

 they appear to resemble most nearly those of the genus Moro- 

 saurus, so well represented in the upper Jurassic of the Rocky 

 Mountain region. A careful comparison, however, shows that 

 they belong to a distinct genus. The teeth are of the same 

 general type as those of Morosaurus, but their crowns are 

 mainly compressed cones, and not spoon-shaped. The dentary 

 bone is slender, and rounded at the symphysis, instead of hav- 

 ing the massive, deep extremity seen in Morosaurus. The 

 maxillary is also lighter, and less robust. The supra-occipital 

 agrees closely in shape with that of Morosaurus, and forms the 

 upper border of the foramen magnum, as in that genus. 



Fig. I. 



Fig. 2. 



Figure 1. Dorsal vertebra of Pleuroccelus nanus, Marsh; side view. 

 Figure 2. The same vertebra ; posterior view. 



Both figures are one-half natural size. 



The cervical and dorsal vertebrae are elongate, and strongly 

 opisthoccelous. The latter are much longer than the corres* 



