the Middle Atlantic Slope. 137 



Whitfield has more recently recognized from the same clays 

 (which he, too, is disposed to refer to the Triassic) five lamel- 

 libranchiates — Astarte veta, Ambonicardia Cookii, Corbicula 

 emaeerata, Corbicula annosa (Conrad's Astarte annosa), and 

 Gnathodon tenuides* In the course of an examination of 

 the formation extending over some years, Fontaine has found 

 but a single animal fossil, viz : the posterior portion of a homo- 

 cereal fish about the size of a salmon from the lower mem- 

 ber on James river. Careful search during the past three 

 months has led to the discovery of moderately abundant dino- 

 saurian remains of upj3er Jurassic type and the upper member 

 between Washington and Baltimore, some of which have al- 

 ready been described by Marsh.f 



At- several points on the Appomattox and James rivers, on 

 both Anna rivers, about Fredericksburg, on Acquia creek, at 

 Baltimore, and at a number of other localities the laminated 

 beds of clay intercalated within the lower member yield abun- 

 dant and well preserved leaf impressions ; and throughout the 

 whole extent of the formation both members abound in silici- 

 fied and lignitized wood. Fontaine has collected and investi- 

 gated the plant remains, and has just sent to press a monograph 

 containing descriptions of over 370 species, of which more than 

 300 are new. Extensive collections of silicified and lignitized 

 wood have also been made from both members and have been 

 investigated by Knowlton ; and eight new species belonging to 

 two new genera have been discriminated and are described and 

 illustrated in a memoir now in press.;}: 



Yiewed as a whole, the Potomac flora is unique. It is, 

 moreover, of special interest in that it records a stage in the de- 

 velopment of plant life not known to be represented elsewhere 

 on the globe. The plant history of the earth falls naturally 

 into two eons, instead of the three represented by animal life, 

 during the first of which the various non-dicotyledonous forms 

 (the cryptogams, cycads, conifers, and monocotyledons) of 

 archaic type prevailed, while throughout the second dicotyle- 

 donous forms of modern type have greatly predominated, — the 

 transition from the archaic to the modern type being sudden, 

 and the bipartite division being consequently stronger and 

 more trenchant than the tripartite division based chiefly on 

 animal remains. Wow the exact period of transition from the 

 archaic flora to the modern one (hitherto placed about the 

 middle of the Cretaceous) § appears to be represented in 

 the Potomac flora ; there is a commingling of primitive and 

 recent types in nearly every plant bearing clay bed ; the types 



* Monog. U. S. Geol. Surv., vol. ix,' 1886, 23-27. 



f This Journal, xxxv, 1888, 89-94. 



X Bulletin U. S. Geol. Surv. (in press). 



§ Ward, 5th Ann. Rep. U. S. Geol. Survey, 1885, Plate lvi. 



