Maryland Geological Survey 553 



ad marginem anteriorem divaricatis ; margine posteriori leviter undata." 

 —Morton, 1827. 



Type Locality.— St. Georges, Chesapeake and Delaware Canal, Dela- 

 ware. 



" Shell of medium size, laterally arcuate. The dimensions of an average 

 specimen are : Length along the arcuate median line from beak to pos- 

 terior extremity, 47 mm. ; distance between beak and posterior extremity, 

 28 mm. ; width of shell at middle, 16 mm.; length of hinge line, 20 mm. 

 Shell usually more or less strongly auriculate, the ears subequal or with 

 one ear somewhat larger than the other. Hinge line straight. Shell 

 marked with from seven to ten deep plications which originate along the 

 lower or convex margin and extend nearly to the beak, not leaving a con- 

 spicuous non-plicate central area, the plications towards the anterior hinge 

 extremity decreasing regularly in size ; along the upper or concave margin 

 the shell is marked by a series of short, marginal plications. Lower valve 

 moderately convex, with a small scar of attachment; upper valve much 

 flatter, its plications similar to those of the lower valve." — Weller, 1907. 



This large, falcate, mesially plicate subspecies of larva has not been 

 found in its typical development within the Maryland lines, although it 

 was collected at a number of localities along the Chesapeake and Delaware 

 Canal. A few small individuals similar in size and shape to the subspecies 

 mesenterica and differing from it only in the persistence of the plications 

 to the umbonal region were collected in Prince George's County. 



Occurrence. — Matawan Formation. St. Georges and Camp Fox, 

 opposite Post 236, Chesapeake and Delaware Canal, Delaware. Mon- 

 mouth Formation. Two miles' west of Delaware City, Delaware ; Brooks 

 estate near Seat Pleasant and Friendly, Prince George's County, Mary- 

 land. 



Collections. — Maryland Geological Survey, Philadelphia Academy of 

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



Outside Distribution. — Matawan Formation. Marshalltown clay marl, 

 New Jersey. Monmouth, Formation. Navesink marl, New Jersey. 

 Widely distributed in the Gulf region but not differentiated. 



