MARYLAND GEOLOGICAL SURVEY 631 



cate; basal margin gently convex, becoming straighter posteriorly; pos- 

 terior margin more narrowly rounded than the anterior. Beaks large and 

 broad, situated from one-fifth to one-fourth the length of the shell from 

 the anterior extremity, strongly incurved and nearly in contact, moder- 

 ately elevated above the hinge line. Valves most prominent at about 

 their mid-height in front of the middle of the shell; from this point the 

 surface curves rather abruptly to the ventral anterior and cardinal mar- 

 gins, and much more gently to the gaping posterior margin ; the cardinal 

 margins back of the beaks are slightly inflected to form a rather distinct, 

 concave cardinal area of moderate width on each valve. Surface of each 

 valve marked by twenty-five or thirty more or less irregular and wavy, 

 rounded, radiating costse of moderate strength, much narrower than the 

 intervening depressions, and closer together in the middle of the shell 

 than at either the anterior or posterior portions ; in the middle of the shell 

 every other costa on large individuals has usually been intercalated 

 between two others at some distance below the beak; the shell is also 

 marked by more or less irregular, concentric undulations. This species 

 is one of the most characteristic members of the Merchantville clay marl 

 fauna, where it sometimes occurs in considerable numbers." Weller, 

 1907. 



The species is not known from Maryland, but it occurs in the form of 

 poorly-preserved casts along the Chesapeake and Delaware Canal in Dela- 

 ware. It is readily recognizable by the well-rounded gibbous valves and 

 the irregular elevated radial lirse. The more southern and apparently 

 later P. Conradi described by Conrad under the name of P. occidentalis 

 has been accepted as a synonym by the later workers, although the two 

 shells are obviously distinct. The northern form runs larger than the 

 southern, is much more nearly cylindrical and less trigonal in outline, the 

 umbones are broader, more evenly rounded, set farther back from the ante- 

 rior extremity and very much less prominent. 



Occurrence. MATAWAX FORMATION. Posts 218 and 105, Briar Point, 

 Post 156, Chesapeake and Delaware Canal, Delaware. 



Collections. Maryland Geological Survey, Philadelphia Academy of 

 Natural Sciences, New Jersey Geological Survey. 



