190 Systematic Paleontology 



of the limbs and feet preserved, agree in general with those of the smaller 

 species of Morosaurus, but indicate an animal of slighter and more 

 graceful build. The metapodials are much more slender, and the 

 phalanges are less robust than in the other members of the order. 



" The known remains of the present species, representing several indi- 

 viduals, indicate an animal not more than 12 or 15 feet in length, and 

 hence the smallest of the Sauropoda. They were found at several local- 

 ities of the Potomac formation in Prince George's County, Maryland. 

 Eegarding the present species as typical, some of the more special char- 

 acters distinguishing these remains from the known Sauropoda are as 

 follows : 



"(1) Teeth with compressed or flattened crowns. 



"(2) Dorsal vertebrae with low neural sutures, and eloiigate excavation 

 in each side of the centrum. 



"(3) Sacral vertebrae solid, with a cavity in each side, and with .face 

 for rib in front. 



"(4) Anterior caudals with flat articular faces, and transversely com- 

 pressed neural spines. 



"(5) Middle caudal vertebrae with neural arch on the front half of 

 the centrum. These characters seem to indicate a distinct family, that 

 may be called the Pleuroccelidm." 



This is by far the best-known species, as nearly all of the more im- 

 portant parts of the skeleton are known, though probably from many 

 different individuals. The known remains are almost entirely those of 

 immature speciinens, but are of comparatively constant relative size, 

 showing probably that the animals they represent were of average size. 

 P. nanus is about the smallest known sauropod; measuring from 12 to 

 15 feet in length by 4 to 5 feet in height, but relatively long of limb, 

 especially in front, as the restoration will show. 



The STcull. — Of the skull the following portions may be recognized: 

 Left dentary, left maxillary, part of right maxillary, left alisphenoid, 

 and minor skull fragments. 



The left dentary is represented by the type specimen No. 5669, U. S. 

 National Museum (pi. xiv, fig. 5). In general form it resembles very 



