MARYLAND GEOLOGICAL SURVEY 663 



Outside Dixtrilmlion. 'Magothy Formation. Cliffwood clay, Xew 

 Jersey. Mataican Formation. Merchantville clay marl, Woodbury clay, 

 Wenonah sand, Xew Jersey. Monmouth Formation. Xavesink marl, 

 Eed Bank sand, Tinton beds, Xew Jersey. 



Superfamily CARDIACEA 

 Family CARDIIDAE 



Genus CARDIUM Linne 

 [Systema Naturae, ed. x, 1758, p. 678] 



Type. Cardium costatum Linne. 



Shell usually subequilateral, closed, or slightly gaping, globose, the 

 united valves subcordate laterally; umbones prominent, almost straight 

 or with a slight anterior twist ; true lunule and escutcheon absent ; sculp- 

 ture dominantly radial ; ribs often granulose, spinose or imbricated ; orna- 

 mentation of lateral areas particularly of the posterior, often differing 

 from that of the disk; ligament external, opisthodetic ; hinge character- 

 ized, with a few exceptions, by two cardinals, of which the ventral is the 

 stronger, and one of two posterior and one of two anterior lateral lamella? 

 in each valve; cardinals more or less twisted; muscle impressions sub- 

 equal; pallial line simple or slightly sinuous posteriorly: internal basal 

 margins serrate. 



The cardiums form a conspicuous element in the faunas of Cretaceous 

 and Tertiary. They are rather fragile, as a rule, and not well adapted to 

 preservation. The external sculpture is frequently formed from a super- 

 ficial shelly layer which readily breaks away leaving no scar upon the 

 polished surface beneath. For this reason it is difficult to tell when one is 

 dealing with a perfectly fresh specimen. The recent representatives, the 

 so-called cockles, number about two hundred species and are most abund- 

 ant in the warmer waters. 



A. External sculpture not spinose. 



1. Altitude of adult shell not exceeding 40 mm.; posterior area flat- 



tened; margins sharply serrate Cardium eufalensc 



2. Altitude of adult shell exceeding 40 mm.; posterior area not flat- 



tened; margins not sharply serrate Cardium spillmani 



Etymology: K apdia, heart. 



