OLDER POTOMAC OF VIRGINIA AND MARYLAND. 525 



Thyksopteris decurrens Fontaine. 



PI. CXI, Fig. 11. 



1889. Thyrsopteris decurrens Font.: Potomac Flora CMonogr. U. S. Geol. Surv., 

 Vol. XV), p. 1.30, pi. xliii, figs. 7, 7a; pi. xlvi, figs. 2, 2a, 4; pi. xlix, figs. 5, 

 5a, 6, 6a, 7. 



Only one specimen of this fern was found at the Langdon locaUty. 

 It occurs in the collection made by Professor Ward in May, 1897. It 

 consists of the upper part of a compound pinna and may be compared 

 with the specimen represented by fig. 7 of pi. xlix of Monograph XV. 

 It is shown on PI. CXI, Fig. 11. This species has a wide range in the 

 Virginia Potomac. 



Zamiopsis insignis Fontaine. 



PI. CXIII, Figs. 4, 5. 



1889. Zamiofsis insignis Font.: Potomac Flora (Monogr. U. S. Geol. Surv., Vol. 

 XV), p. 162, pi. Ixii, fig. .3; pi. Ixiv, figs. 1, 3; pi. Ixv, figs. 4-6; pi. Ixvi, 

 fig. 2; pi. Ixvii, fig. 7. 



Two impressions of a plant were found that belong to Zamiopsis 

 insignis. This remarkable plant is highly characteristic of the lowest 

 strata of the Lower Potomac of Virginia. At the time of the preparation 

 of Monograph XV it had been found only at the Fredericksburg locality, 

 but since that time it has been discovered in abundance near Alexandria, 

 in the basal beds at Chinkapin Hollow. This is a type that is easily 

 detected even in small fragments. The specimens show portions of 

 ultimate pinn«. The form given in PI. CXIII, Fig. 4, comes from lower 

 down on the plant, where the pinnules are larger. Several of these are 

 shown. The form given in fig. 5 is the terminal portion of an ultimate 

 pinna; where the pinnules are narrow and show only the characteristic 

 toothing of the plant. The distribution of this species is geologically so 

 well defined that the plant is especially valuable to fix the horizon of 

 the strata in which it occurs. 



Both the specimens were found by Professor Ward in May, 1897. 



GENERAL REMARKS. 



The account of the plants given above makes it evident that the 

 locality yielding them is on the horizon of the basal beds of the Potomac 



