Maryland Geological Survey 67 



oda there are two, one the nnarmored Dryosaunis, tlie other, Priconodon, 

 evidently belonging to the armored group or Stegosauria. 



The dinosaurs show none of the remarkable over-specialization of the 

 later types, but, on the contrary, represent the order at the crest of the 

 evolutionary wave, before the signs of decadence set in. Unfortunately, 

 owing to an almost utter dearth of terrestrial Jurassic deposits, nothing 

 is known of dinosaurian evolution in America from Newark time until 

 we come to the horizon under consideration. In Europe the record, 

 though still meagre, is more complete ; but it represents in every instance 

 more primitive types than those of the Potomac and the Morrison. 



The flora is of the same type as that of the Patuxent, most of 

 the genera and a large number of the species of the latter having been 

 found in the Arundel and where unknown the presumption is strong 

 that they still existed in nearby areas, since the known Arundel flora 

 contains no new or younger elements than does the Patuxent, and indi- 

 cates that the marked change in the flora of the Potomac occurred during 

 the time interval represented by the unconformity between the Arundel 

 and the overlying Patapsco formation. 



The Arundel formation also contains poorly preserved representatives 

 of fresh-water molluscs. 



THE patapsco FGRMATIOISr 



ISTame and Synonymy. — The Patapsco formation was named by 

 Clark and Bibbins ^ from the Patapsco Eiver in Maryland, in the drain- 

 age basin of which stream the deposits are well exposed and were first 

 studied as an independent formation. It was included by Eogers in his 

 "Upper Secondary" or " Jurasso-Cretaceous"; by Tyson together with 

 the preceding formation in his " Upper oolite." It was with the Arundel 

 included by McGee in his upper or " varicolored " clay member. The 

 formation was not differentiated either by Marsh, Fontaine, Ward, or 

 Darton in their Potomac. It corresponds in part to what Fontaine 

 termed the Baltimore beds, and includes Ward's Mt. Vernon series and 



^Loc. cit., p. 489. 



