MAltYLAM) GEOLOGICAL SUUVEY 195 



truncated in the center by a rectilinoar line; cavity capacious; margin 

 crenate. Length, 1| in." Conrad, 1833. 



Shell moderately thick, ()I)!i(|uely oi'ljicnlai'. latluT vcntricose, slightly 

 iiu'ijuilateral, eciuivalvc, willi Taint radiations; hinge with few promi- 

 nent teeth; margins crcnulated; beaks depressed. 



The higher beds of the Aquia formation, especially the ind mated 

 ledge at Up])er Marlboro and at the localities to the northeast, are very 

 rich in shells of Glycymeris. Altlioiigh in some features they at times 

 show slight variations from G. idoneus of the Gulf, the differences are 

 too slight to warrant specific distinction. 



Length, 60 mm.; height, 60 mm. 



Occurrence. — Xaxjemot Foe:^iation'. 3 miles above Popes Creek, 

 AYoodstock. Aquia Fokmatiox. Upper Marlboro, 1 mile west of 

 Hardesty, Potomac Creek, 1 mile northeast of Piscataway, Hardesty, 

 Sheckels Farm near South Eiver, 2 and 3 miles south of South Eiver, 

 Eolph's Landing. 



Collections. — Philadelphia Academy of jSTatural Sciences, Johns Hop- 

 kins University, Maryland Geological Survey. 



Family LIMOPSIDAE. 



Genus TRIGONOARCA Conrad. 



Trigo^'Oaeca decisa (Conrad) yae. 



Plate LI, Figs. 5, 5a. 



Pectunculus decisus Conrad, 1833, Fossil Shells of the Tertiary, p. 39. 



Limopsis decisus Conrad, 1860, Jonr. Acad. Nat. Sci. Phila., 2nd ser., vol. iv, p. 297, 



pi. xItII, fig. 13. 

 (?) Xoetia pulchra Gabb, 1860, Jour. Acad. Nat. Sci. Phila., 2nd ser., vol. iv, 



p. 388, pi. Ixvii, tig. 65. 

 Trigonoarca pulchra var. Harris, 1897, Bull. Amer. Pal. No. 9, p. 48, pl. viii, tigs. 



3, 2a. 



Description. — " Shell longitudinally suboval, equilateral, with obsolete 

 radiating striae; posterior end obliquely truncated, umbonial slope angu- 

 lated, incurved; beaks small, pointed and recurved; cardinal teeth small 

 and crowded; series not much arcuated; margin entire. Length, ^ of an 

 inch.-"' Conrad, 1833. 



This form is smaller and proportionally longer than T. pulclira Gabb 



