174 The Eeptilia of the Arundel Formation 



Another, much larger form, seems to have been present, though leaving 

 the most meagre record of its existence in the shape of a single verte- 

 bra — evidently that of a Creosaurus, to which the name of potens, the 

 powerful, is given. This animaf must have resembled Allosaurus very 

 closely. 



Beside the huge carnivores, there were lesser ones of the most delicate 

 build, with bones lightened by the development of interior spaces, doubt- 

 less filled with air, as in the birds, until an almost paper-like thinness 

 of the bone resulted. The Maryland animal, known only from the teeth 

 and a single claw, resembled in tooth and claw its western allies. The 

 latter Professor Marsh called Ccelurus, hollow tail, in allusion to the 

 pneumatic bones, and to the Arundel species he gave the name of 

 Ccelurus gracilis. This animal must have been extremely active, preying 

 upon feebler reptiles and possibly upon the primitive mammals and birds 

 of that day. 



Of the plant-feeding dinosaurs, the sub-order of great quadrupeds, the 

 Sauropoda are represented by at least one genus, Pleuroccelus, possibly 

 by others; but this is not certain. 



Pleuroccelus is a long-legged type, apparently of small size, though 

 most of the individuals represented were immature at the time of their 

 death, though, strangely enough, of fairly constant size, which seems 

 to imply either a wholesale slaughter of the young or that they really 

 had reached the average stature, though not full maturity. Of these, 

 two species are represented, one Pleuroccelus nanus, the restoration of 

 which is shown on pi. xi, was but 12 feet or so in length by 4 feet 

 in height; a pigmy compared wdth the colossal bulk of Brontosaurus of 

 the West with its length of over 60 feet. 



Pleuroccelus nanus is by far the best known of the Potomac dinosaurs' 

 for more than half of all the material collected may be attributed to it. 

 It was lightly built, though, in common with its allies, this lightness was 

 largely confined to the skull and backbone, while the limb bones were 

 much more solid in texture. Pleuroccelus resembles most closely the 

 form known as Morosaurus, a genus of wide distribution, from which it 

 differed mainly in the greater relative length of the fore limbs and of 



