Maryland Geological Survey 533 



Shell rather small and thin for the genus, moderately inflated, rudely 

 trapezoidal in outline : umbones evenly inflated, feebly prosogyrate, proxi- 

 mate, placed a distance of one-third to one-half of the total latitude back 

 from the anterior margin; posterior area flattened, posterior keel obtuse; 

 anterior lateral margin uniting with the hinge line at an angle of approxi- 

 mately 110°. merging into the basal margin with a very broad and sweep- 

 ing curve; posterior lateral margin obliquely truncated, ventral margin 

 horizontal medially or somewhat obliquely arcuate ; external surface 

 smooth, excepting for a microscopically fine radial lineation and a rather 

 feeble incremental sculpture, least so upon the anterior portion, and a 

 rather broad but shallow sulcus which follows the median line of the 

 posterior area, becoming more feeble dorsally and gradually evanescing at 

 the umbonal region, corresponding in position to the inner buttress of the 

 posterior adductor: cardinal area rather low, rhomboidal, grooved with 

 three or four concentric, diamond-shaped sidei ; hinge plate presenting a 

 straight dorsal margin, feebly arcuate ventrally : hinge teeth short, similar, 

 transverse to the hinge line medially, approximately six in number, in the 

 adult forms much coarser and slightly oblique or parallel to the hinge line 

 distally ; anterior muscle scar indistinct, posterior conspicuously buttressed 

 on its inner margin : pallial line entire ; inner margins not crenulated. 



The casts, the form in which the species most commonly occurs in 

 Maryland, are characterized by the evenly rounded outline, the maximum 

 diameter falling not far from the median horizontal, the broad, low, 

 proximate umbones and the slight obliquity of the basal and posterior 

 lateral margins. Even the young of C. vulgaris are strongly oblique when 

 occurring in the form of casts and their umbones are very much higher, 

 narrower and more compressed. The shells of C. otrolinensis and the 

 young of C. vulgaris are quite similar in outline, but the former are much 

 heavier and are more strongly sculptured radially. 



Occurrence — Matawan Formation. Camp U & I. Chesapeake and 

 Delaware Canal. Delaware : Arnold Point. Anne Arundel County. Mary- 

 land. Monmouth Formation. Briar Point. Chesapeake and Delaware 

 Canal. Delaware: Cayots Corners and Bohemia Mills. Cecil County: 

 Brightseat and Fort Washington, Prince George's County. Maryland. 



