112 Marsh — Jurassic Formation on the Atlantic Coast. 



The genera named in my recent classification were mainly 

 typical forms, and I had no intention of making a complete 

 catalogue of all the known genera, as anyone familiar with the 

 subject could readily see. By way of further instruction, let 

 me repeat here what I have recently said about one of these 

 typical forms. 



" Pleurocoelus is one of the most characteristic genera of the 

 Sauropodous Dinosaaria, and its value in marking a geological 

 horizon should therefore have considerable weight. It is now 

 known from the two European localities mentioned above, both 

 in strata of undoubted Jurassic age. The same genus is well 

 represented in the Potomac deposits of Maryland, and has been 

 found, also, in the Atlantosaurus beds of Wyoming, thus offering, 

 with the associated fossils, strong testimony that the American 

 and European localities are in the same general horizon of the 

 upper Jurassic."* 



Had Mr. Gilbert been familiar with the subject discussed in 

 his review, he would have known that, so far as present evi- 

 dence goes, there are other genera of Dinosaurs common to 

 Europe and America, found in apparently the same Jurassic 

 horizon, and that this is true also of various other reptiles and 

 of fishes. More important still is the correspondence between 

 the genera of Jurassic mammals of the two continents, which 

 in itself is sufficient to demonstrate that they belong in essen- 

 tially the same horizon. 



The last point Mr. Gilbert raises in his review is a geologi- 

 cal one, and even here he has missed the mark. His words 

 are as follows : 



"The physical relations of the beds afford a presumption in 

 favor of their Cretaceous age. Prof. Marsh mentions that the 

 Potomac formation in New Jersey passes by insensible gradation 

 into marine Cretaceous above." 



The two statements in this quotation are, in my opinion, 

 both erroneous, and the second is contrary to the idea I 

 intended to convey. The physical relations of the beds in 

 question are in favor of their Jurassic age, and the Potomac 

 strata do not pass by insensible gradation into the marine Cre- 

 taceous above. Although the two are apparently conformable, 

 the passage from one to the other was a change from fresh- water 

 to marine deposits, which in itself implies a break that may 

 represent a long period of time, perhaps the entire lower Cre- 

 taceous. This break was clearly indicated in the geological 

 section that I gave in my paper (figure 2),* and to make this 

 point clear, the same section is here repeated. 



*-Ttis Journal, vol. iv, p. 415, December, 1897. 



