518 Systematic Paleontology 



The species may be properly referable to Yoldia, but the prominence of 

 the rostral ray and the well differentiated lunule and escutcheon are much 

 more suggestive of Leda. It is quite probable, too, that Leda rostra- 

 truncata may be identical with some of the numerous casts described from 

 the New Jersey Cretaceous, but none of them exhibit the squarely trun- 

 cated posterior keel set off by the contraction of the base line in front of 

 it, a character which is so conspicuous in Leda rostratruncata. The form 

 is the most abundant representative of the genus in the Monmouth of 

 Prince George's County. 



Occurrence. — Monmouth Formation. Brightseat, railroad cut west 

 of Seat Pleasant, Brooks estate near Seat Pleasant, Friendly, 1 mile west 

 of Friendly, 2 miles south of Oxon Hill, Fort Washington, Prince George's 

 County. 



Collection. — Maryland Geological Survey. 



Genus YOLDIA Moeller 

 [Index Moll. Groenl., 1842, p. 18] 



Type. — Yoldia arctica Gray. 



The genus differs from Leda mainly in the posterior gape of the valves 

 and the much deeper pallial sinus, resulting from the longer siphons. 



A. Posterior dorsal margin approximately horizontal, and sub-parallel to 



the straight base line Yoldia longifrons 



B. Posterior dorsal margin excated or oblique, base line more or less 



arcuate. 



1. Posterior dorsal margain excavated, umbones compressed. 



Yoldia gahbana 



2. Posterior dorsal margin oblique, umbones moderately inflated. 



Yoldia noxontownensis 



Yoldia longifrons (Conrad) Johnson 

 Plate XIX, Fig. 13 



Leda longifrons Conrad, 1860, Jour. Acad. Nat. Sci., Phila., 2d ser., vol. iv, 



p. 281, pi. xlvi, fig. 18. 

 Nuculana longifrons Meek, 1864, Check List Inv. Foss. N. A., Cret. and Jur., 



p. 8. 

 Nuculana longifrons Whitfield, 1885, Mon. U. S. Geol. Survey, vol. ix, p. 107, 



pi. xi, figs. 16, 17. 



Etymology: Dedicated to the Count of Yoldi. 



