188 Systematic Paleontology 



the border is broken away where the crennlations would occur if present. 

 The curvature of the teeth is similar in each species, while the variation 

 in size may be accounted for by a difference in the stage of growth of 

 the individual teeth ; as it is, two of the Maryland ones are slightly larger 

 than the tooth of Ccelurus fragilis figured by Marsh. 



Occurrence. — Aeundel Foemation. Near Muirkirk, Prince George's 

 County, 



Collections. — ^U. S. National Museum, Goucher College. 



Suborder SAUROPODA 

 Family MOROSAURIDAE 



Genus PLEUROCOELUS Marsh 



Pleuroccelus nanus Marsh 



Plate XI; Plate XIY, Figs. 5-8; Plate XV, Figs. 2-5; Plate XYI; 



Plate XVII; Plate XVIII, Figs. 1, 2 



Pleuroccelus nanus Marsh, 1888, Amer. Jour. Sci., ser. iii, vol. xxxv, p. 90, 



figs. 1-6. 

 Pleuroccelus nanus Lydekker, 1890, Catal. of Fossil Reptilia and Amphibia 



in the British Museum, pt. iv, p. 238, fig. 52. 

 Pleuroccelus nanus Marsh, 1896, 16th Ann. Rept. U. S. Geol. Survey, pt. i, 



pis. xl, xli. 

 Pleuroccelus nanus. Hay, 1902, Bull. U. S. Geol. Survey, No. 179, p. 483. 



Description. — Type material consists of a cervical vertebra (No. 5678), 

 a dorsal (No. 4968), a sacral (No. 4969), a caudal (No. 4970), all of 

 the U. S. National Museum collection. In addition to these, a caudal 

 vertebra (No. 3372), four metatarsals, and two unguals (No. 2667), a 

 supra-occipital bone (No. 5692), a left dentary (No. 5669), and a tooth 

 (No. 5691) are also in the National Museum. 



Marsh's original description of this species is as follows : " 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 they agree so nearly that they may be referred 



