MARYLAND GEOLOGICAL SURVEY 321 



Description. — " Suborbicular, ventricose ; ribs about 29, very much 

 compressed, profoundly elevated, the summits reflected on both sides, 

 consequently the ribs are as wide on the back as at base; summit of the 

 umbo very prominent. 



" Eemarkable for the compressed form and great elevation of the ribs 

 which are most remote on the anterior side; ribs not very regular, but 

 somewhat sinuous." Conrad, 1845. 



Ribs sometimes as many as thirty-two, in transverse section angular, 

 very narrow, highly elevated, and wider on top than just beneath; dis- 

 tance from beak to base proportionally greater as compared with length 

 than in C. leptopleurum ; hinge line proportionally short; shell thin, easily 

 broken; beaks approximate. 



Length, 52 mm. ; height, 62 mm. ; diameter, 18 mm. 



Occurrence. — Calvert Formation. 3 miles south of Chesapeake 

 Beach, Plum Point (rare). 



Collection. — Maryland Geological Survey. 



Cardium (Cerastoderma) calvertensium n. sp. 

 Plate LXXXAa, Fig. 4. 



Description. — Shell elevated, rounded; beak prominent; ribs seventeen 

 to twenty-two, rounded, rather distant, the interspaces as broad as, or 

 broader than, the ribs; on anterior and posterior dorsal submargin ribs 

 small to almost obsolete ; entire interior of shell strongly grooved in har- 

 mony with the ribbing; cardinal and lateral teeth small, not prominent. 



It may be readily distinguished from other species by its much fewer 

 ribs, rounded form, strong internal grooving and small teeth. It is 

 abundant as casts in the basal clays, but is very rare in the later deposits 

 of the Miocene. 



Length of shell broken along growth line, 37 mm.; height, 38 mm.; 

 diameter, 14 mm. 



Occurrence. — Calvert Formation. Fairhaven, Plum Point, Gov- 

 ernor Eun (at base of cliff). White's Landing. 



Collection. — Maryland Geological Survey. 



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