26 
MACTRA DELUMBIS. Tab. 11. 
Suboval, thin and fragile, with a fold on the posterior sub- 
margin; umbo prominent; beaks nearly central, approximate; 
lunule much elongated, lanceolate, slightly impressed. 
Localities . James River, near Smithfield; Suffolk, Ya. Up- 
per Tertiary. 
This fine species is not uncommon in the blue marl of certain 
districts, but is so extremely fragile that entire valves can rare- 
ly be obtained. In the banks of St. Mary’s river, Maryland, 
a Mactra has been discovered, which, so far as a very imperfect 
valve admits of determining, appears to be identical with the 
present species. 
PHOLADOMYA ABRUPTA. Tab. 12. 
Oblong oval, much compressed, with from three to five sub- 
acute distant ribs or ridges diverging from the apex; one side 
rather thick and strong, rounded at the extremity; the opposite 
side extremely thin, and reflected, with a truncated margin; 
muscular and palleal impressions distinct. 
Locality . York-town, Ya. Upper Tertiary. 
This rare shell has been found only in the locality above men- 
tioned, where I procured two imperfect valves, and Mr. Finch 
informs me that he also obtained a broken specimen. 
The genus Pholcidomya embraces but few species, and they 
chiefly belong to Secondary formations. M. Marcel de Serres, 
however, has detected this genus in the tertiary marls of the 
South of France, and one recent species is known, which is 
figured in Sowerby’s Genera of Shells. 
