MARYLAND GEOLOGICAL SURVEY 403 



" Suborbicular, irregular, with radiating rugose lines, obsolete or 

 AA anting except on the space anterior to the apex, where they are distinct. 



" Var. A. Apex remote from the margin ; lines distinct over the whole 

 disk, and reticulated b}^ fine wrinkles." Conrad, 18^5. 



'• Suboval, compressed; surface uneven, with radiating rugose closely- 

 arranged lines." Conrad, 184.5. (Description of Orbicnla multilineata.) 



Dorsal valve subcircular to oval, usually deep but sometimes almost 

 flat; substance cornaceous, thick; apex prominent, varying in position 

 from terminal to very slightly eccentric, nearer the center in the thinner, 

 flatter, more nearly circular valves; external surface with very distinct 

 concentric lines of growth, and with very variable radiating vermicular 

 threads which are often obsolete on part or all of the valve; muscular 

 scars very prominent; anterior scars large, elongate, not varying much 

 m width from one end to the other, nearest together at the center of the 

 valve, increasing in distance posteriorly; posterior scars small, distant, 

 very prominent, somewhat elevated : ventral valve thin, flat or shaped by 

 the surface to which it adheres; septum small, narrow, elevated; longi- 

 tudinal fissure small, broad. 



Dorsal valves are very abundant as their composition makes them more 

 easily preserved than any other species in this fauna. Ventral valves 

 are, however, very rare. They are so thin that they are never preserved, 

 except when they have remained attached to some foreign object with the 

 dorsal valve still in position. The ventral valve here figured, which is 

 apparently the only one which has been found in Maryland, was attached 

 to the exterior of a large Ecphora quadricostata. 



There is great variation within this species due chiefly to the object 

 of attachment and other conditions of environment. In addition to the 

 typical form which was descriljed from Stow Creek, New Jersey, and 

 which was characterized by having semiobsolete radiating lines and a 

 marginal apex, Conrad recognized an unnamed variety, from St. Mary's 

 county (probably from Jones Wharf), which had a subcentral apex and 

 distinct radiating lines, and a supposedly distinct species multilineata 

 which had a depressed dorsal valve with a subcentral apex and very 

 rugose radiating sculpture. 



As has already been pointed out, the thickness of the shell and the 



