584 SYSTEMATIC PALEONTOLOGY 



This species is the smallest and most abundant member of this remark- 

 able genus within the confines of Maryland. It is separated from 

 T. cerulea Whitfield by the more prominent umbones, the more convex 

 posterior dorsal, the more attenuated posterior extremity and the fewer 

 rugose and relatively coarser external costae. 



Occurrence. MONMOUTH FOTIMATIOX. ? 2 miles west of Delaware 

 City on John Higgins farm, Delaware ; ? Bohemia Mills, Cecil County ; 

 mouth of Turner's Creek, Kent County; Brightseat, Brooks estate near 

 Seat Pleasant, Friendly, 1 mile west of Friendly, McNeys Corners, Fort 

 Washington, Prince George's County, Maryland. 



Collections. Maryland Geological Survey, Philadelphia Academy of 

 Natural Sciences, New Jersey Geological Survey, U. S. National Museum. 



Outside Distribution. Matawan Formation. Merchantville clay marl, 

 Woodbury clay and Wenonah sand, New Jersey. Black Creek Formation. 

 North and South Carolina. Peedee Sand. North and South Carolina. 

 Eutaw Formation (Tombigbee sand member). Exogyra ponderosa zone, 

 Mortoniceras subzone, Georgia. Ripley Formation. Exogyra costata 

 zone, Georgia ; Eufaula, Alabama. Extreme top of zone, Pataula Creek, 

 Georgia. 



TRIGONIA CERULEA Whitfield 



Trigonia cerulea Whitfield, 1885, Mon. U. S. Geol. Survey, vol. ix, p. 114, pi. 



xiv, fig. 7. 

 Trigonia cerulea Weller, 1907, Geol. Survey of New Jersey, Pal., vol. iv, p. 



464, pi. xlviii, fig. 13. 



Description. " Shell small or below a medium size, moderately convex 

 on the valves and of a triangularly-ovate outline. Beak small, appressed, 

 obtusely pointed and erect; posterior hinge-line long and slightly con- 

 cave ; posterior end narrow and rounded ; anterior end broadly rounded ; 

 basal line a litle gibbous in the middle, but otherwise forming a continuous 

 line with the anterior and posterior margins. Surface of the shell cov- 

 ered by coarse elevated ribs, which are flattened on their surfaces over a 

 large part of the shell, but near the posterior cardinal margin are sharp 

 and very slightly crenulated. The ribs are coarse and distant on the ante- 

 rior and middle parts of the shell, but gradually become finer and more 



