86 CONTRIBUTIONS 
exists in Derbyshire in the first mentioned formation, and 
twenty-seven others in the superior beds of England. Five 
of these are from the London Clay. M. Deshayes gives 
us twenty-three from the Tertiary of Europe. Six of these 
are from the Paris basin. Mr Say describes two species, 
the levis and concentrica from the Tertiary of Maryland, 
and Mr Conrad two from the Tertiary of Claiborne. 
ORDER CONCHIFERA UNIMUSCU- 
LOSA. 
FAMILY MALLACEA. 
GENUS AVICULA. Lamarck. 
4. Claibornensis. Plate 3. Fig. 65. 
Description. Shell thin, thickened along the dorsal 
margin, acutely angular before ; beaks small, pointed ; 
teeth small and placed near the anterior margin; wings 
very oblique ; nacre very pearly. 
IDF eNOS Gore Length.... Breadth... . 
Observations. Several fragments and one nearly com- 
plete valve only were received by me. From these, I am 
unable to identify it with any other species. It differs 
from the 4. media (Sowerby), in being more oblique and 
