MARYLAND GEOLOGICAL SURVEY 723 



the cardinal margins, rounding smoothly into the anterior lateral margin 

 and more abruptly into the posterior ; external surface undulated concen- 

 trically, rather sharply and regularly in the immediate vicinity of the 

 umbones, broadly and irregularly over the medial portion and more closely 

 toward the ventral and lateral margins ; ligament external, opisthodetic, 

 mounted on a short but very robust nymph; no hinge plate developed, 

 armature restricted to a single cardinal tooth in each valve, the cardinal of 

 the right valve in front of that of the left when interlocked ; characters of 

 muscle scar and pallial line unknown. 



Dimensions. Altitude 65.6 mm., latitude 100.5 mm., semi-diameter 

 19 mm. 



Type Locality. Brightseat, Prince George's County. 



The form differs from Conrad's decisa in being more compressed, with 

 less prominent, more central umbones and a more constricted anterior 

 dorsal margin. 



Occurrence. MOXMOUTH FORMATION. John Higgins farm, 2 miles 

 west of Delaware City, Delaware ; ? Cayots Corners, Cecil County ; Bright- 

 seat, Brooks estate near Seat Pleasant, ? McNeys Corners, Prince George's 

 County, Maryland. 



Collections. Maryland Geological Survey, Philadelphia Academy of 

 Xatural Sciences, Xew Jersey Geological Survey, U. S. National Museum. 



PANOPE BONASPES n. sp. 

 Plate XLV, Fig. 2 



Description. Shell rather small, elliptical in outline, almost twice as 

 high as wide, moderately and evenly inflated ; umbones compressed ; sub- 

 central, overtopping the dorsal margin; anterior portion of shell sym- 

 metrically rounded, the posterior slightly wider and more broadly arcuate ; 

 base line approximately straight ; external surface sculptured with irregu- 

 lar, concentric corrugations which are most pronounced on the medial 

 dorsal and posterior portions of the shell, evanescing anteriorly and 

 broader and more shallow ventrally; characters of interior of shell not 

 known. 



