MARYLAND GEOLOGICAL SURVEY 585 



closely arranged toward the posterior part. Interspaces concave. No 

 postero-cardinal area is visible on the specimen used, the ribs apparently 

 passing, without interruption, across the entire disk of the shell and ter- 

 minating on the cardinal margin. The ribs of the anterior end curve 

 strongly forward in passing to the basal and anterior margins, while those 

 of the hinder parts of the valves pass more directly across to the postero- 

 basal margin. This species differs from any of the others described 

 from these beds in its form, but more particularly in the style and number 

 of the surface ribs. They are more numerous than on any of the other 

 forms, there having been about twenty-three on the specimen figured, 

 which is only one inch and an eighth in length. Their flattened surface 

 and the gradual increase backward is also opposite from that which is seen 

 to occur on those .... In coarse olive-green indurated marl at the deep 

 cut on the Holmdel and Keyport Turnpike, Monmouth County, New 

 Jersey, at the base of the Lower Marls. The substance of the shell is 

 entirely changed to vivianite, which is soft and of a bright blue color, very 

 easily destroyed by handling or rubbing." Whitfield, 1885. 



This species is represented in the collections of the Maryland Cretaceous 

 by a single valve. The species is somewhat larger than the more common 

 T. eufalensis Gabb, is less trigonal and more semi-elliptical in outline, not 

 rostrate posteriorly and less coarsely and more uniformly sculptured. 



Occurrence. MONMOUTH FORMATION. Brooks estate near Seat Pleas- 

 ant, Prince George's County. 



Collections. Maryland Geological Survey, Columbia University, New 

 Jersey Geological Survey. 



Outside Distribution. Monmouth Formation. Tinton beds, New 

 Jersey. 



TRIGONIA MARIONENSIS Stephenson n. sp. 



Description. " Shell subtrigonal, equivalve, inequilateral, moderately 

 ventricose anteriorly, becoming strongly compressed posteriorly; beak 

 small, incurved, situated about one-quarter the length of the shell from 

 the anterior extremity. Hinge and dorsal area too poorly preserved for 

 description. 



