176 The Eeptilia op the Aeundel Formation 



Dryosaurus could rest upon all fours while feeding, using the bird-like 

 hind limbs alone when in motion. Dryosaurus, a close ally of Laosaurus, 

 the restoration of which is shown on pi. xiii, is found in the Morri- 

 son beds of the West in the form of a nearly related species, Dryosaurus 

 alius. To the Potomac form, because of its greater size, among other 

 distinctive features, the name of Dryosaurus grandis is given. The 

 nearest European ally to the present species is Hypsilophodon foxii from 

 the English Weal den beds, smaller and in some respects more primitive 

 than the American types and representing a collateral line of descent. 



The larger, more heavily built predentates of the Morrison and of 

 Europe have not as yet been discovered in the Arundel deposits. This, 

 however, is not surprising, for their habits of life differed so essentially 

 from those of the Sauropoda that their remains are rarely found asso- 

 ciated in the roeks Another locality for Potomac dinosaurs might 

 readily disclose these larger terrestrial types. 



There is another predentate type, known from a number of similar 

 teeth, to which the name of Priconodon crassus has been given by 

 Marsh. These teeth indicate a relationship with the armored dinosaurs 

 or Stegosauria, forms which, because of the increasing weight of their 

 protective mail, came down on all fours, not only for resting but for 

 actual locomotion, as the increasing power of the fore limbs gives evi- 

 dence. Judging from the teeth alone the nearest ally of Priconodon is 

 Palceoscincus costatus described by Leidy from the Judith Eiver beds 

 of Montana, which are of Upper Cretaceous age. The teeth of the known 

 contemporary (Stegosaurus) are much smaller and of a more specialized 

 character. Stegosaurus, however, is known to be a remarkably specialized 

 side line, probably the terminal member of its race, as its remains are 

 unknown above the Morrison formation. There were, however, more 

 conservative armored types which persisted until the close of the age 

 of reptiles, and to this race Priconodon seems to have belonged. One 

 vertebral centrum has been found in the Arundel beds very similar 

 to that of Stegosaurus, except that it is smaller, but, as it represents an 

 immature animal, that distinction has but little weight. It has been 



