662 SYSTEMATIC PALEONTOLOGY 



apices, proximate, incurved and prosogyrate, placed a little in front of the 

 median vertical ; anterior and ventral margins well rounded, merging 

 gradually into one another, the outline of the posterior margin sometimes 

 rounded, sometimes obscurely truncate obliquely ; external surface smooth 

 excepting for faint incremental striations which are least feeble near the 

 posterior extremity; ligament opisthodetic, lodged in a submarginal 

 groove extending backward for some distance from the tips of the 

 umbones ; hinge plate narrow, very fragile ; armature in the right valve 

 consisting of a thin, laminar, hook-shaped cardinal directly beneath the 

 umbones, its posterior arm vertically directed, its anterior arm approxi- 

 mately horizontal ; right posterior cardinal slender, laminar, obliquely 

 elongated, parallel to the dorsal margin ; anterior lateral developed as a 

 thin plate proximate to and directly facing the anterior portion of the 

 hooked cardinal; hinge of left valve consisting of an anterior A -shaped 

 cardinal which fits between the left anterior lateral and the vertical arm of 

 the cardinal hook of the right valve; a very thin, laminar, medial cardinal 

 which is accommodated between the anterior lateral and the horizontal 

 arm of the hook, and a thin, laminar, obliquely elongated, posterior car- 

 dinal; muscle scars small, not very distinct, placed high up near the distal 

 extremities of the dorsal margins ; pallial sinus narrow, but quite deep, 

 steeply ascending; inner margins simple. 



This species occurs very abundantly in the form of casts in the Mon- 

 mouth of Prince George's County, but the shell is so exceedingly thin and 

 so readily flaked off that it is seldom possible to secure a perfect indi- 

 vidual. 



Occurrence. MAGOTHY FORMATION. Good Hope Hill, District of 

 Columbia. MAT AW AX FORMATION. Post 105, Chesapeake and Delaware 

 Canal, Delaware; ? Ulmstead Point, Anne Arundel County, Maryland. 

 MONMOUTH FORMATION. Fredericktown, Cecil County ; Brightseat, 

 Brooks estate near Seat Pleasant, McNeys Corners, Friendly, 2 miles south 

 of 0-xon Hill, ? Fort Washington, Prince George's County, Maryland. 



Collections. Maryland Geological Survey, Philadelphia Academy of 

 Natural Sciences, Xew Jersey Geological Survey. 



