e 



AGE OF THE POTOMAC GROUP 337 



tially the sanu' surviving g-eneric types from the Jurassic, the same new 

 generic types, hoth those peculiar to the Lower ('retaceoiis and tliose fore- 

 shadowing later Horas, and a similar specific variation in the genera. 

 The common genera are numerous; the common species are more nu- 

 merous than might be expected, and finally the closely allied species are 

 very numerous. 



One additional fact should be emphasized in connection with the flora 

 of the Potomac group, since it has an important bearing on the age of 

 the Morrison fonnation. This is the essential unity of the floras from 

 both the Patuxent and Arundel formations and their contrast with the 

 flora of the overlying Patapsco formation. An analysis of these three 

 floras develops the fact that the Arundel formation has only furnished 

 five species (in five genera) not present in the underlying Patuxent, and 

 four of these five genera are represented by closely allied Patuxent s])ecies, 

 while the fifth has closely allied forms in the Barremian and Ajjfian of 

 Europe. 



The Potonuic reptilian fauna, which all vertebrate paleontologists have 

 compared with that of the Morrison, is found in the Arundel formation 

 unconformably overlying the Patnxent formation, which is from 200 to 

 400 feet in thickness. The hist authoritative study of the Arundel fauna 

 was that of Lull, ])ul)lished in IIMI. The following conclusions are 

 quoted fi'niii his (•(Hiti'ihiitioii."' 



"The fossil reptiles of the Potomac, while not so abundant in numbers or 

 kinds as in tlie Morrison of our Western States, nevertlieless compare very 

 closely with the latter, as nearly all of the Potomac genera and, in some in- 

 stances, very closely allied if not identical species are foinid in the West. 



"A striking similarity also iirevails ln'tween the Potomac on the one hand 

 and the Wealden of Euroi)e on the othei-. while one important Maryland genus 

 is reported from a lower horizon than the Wealden and none from a higher 

 level. 



"The dinosaurs represent all of tlie suborders, including two of the heavier, 

 megalosaurian carnivores, AUomurus and Creosaurus, and one of the lighter. 

 Compsognathoid type. Cahirus. The quadrupedal Sauroi)oda are represented 

 by at least one genus. ])ossibly two, /'Iciirocalus and AstrodoH, including two 

 or three species in all. while of the Orthopoda there are two, one the unar- 

 mored Drj/oxuurus, the other, I'rirotKxhui, evidently belonging to the armored 

 grou]) or Stegosauria. 



"Tlie dinosaurs show none of the remarkabl(> over-specialization of the later 

 Cretaceous tyjies. but. on the contrary, reitresent the oi'der at the crest of the 

 evoluti(»nary wave before signs of decadence set in. Unfortunately, owing to 

 an almost utter d(>arth of terrestrial .Jurassic deposits, nothing is known ot 

 diiiosain-ian evohUioii in America from Newark time until we come to the 



* K. S. Lull: The roptilia of lln' .\niinlcl l"<prmnti(in. Maryland Ccol. Siirvi'V. T.owor 

 C'rctaccoiis. liHl, pii. tT.'i-lTs. 



