212 CONTRIBUTIONS 
from the examination of that celebrated deposit, about 
nine years since. 
In Great Britain three species of Balanus have been 
observed, all in the Crag. I am not aware that any 
species of this genus has been before described from our 
Formations. 
MACTRA. Linneus. 
MV. clathrodon. Plate 6. Fig. 223. 
Description. Shell subtriangular, thin, inequilateral, 
obscurely and transversely striate ; beaks somewhat point- 
ed; lateral teeth crossed by equidistant minute striz ; 
excavation of the palleal impression small and rounded ; 
anterior and posterior cicatrices scarcely visible ; cavity of 
the shell somewhat deep ; cavity of the beaks rather deep. 
Diam. .2, Length 5-20ths, Breadth 7-20ths, of an inch. 
St Mary’s, Maryland. John Finch. 
Deal, New Jersey.* 
Observations. In outline this species is somewhat like 
M. Grayi (nobis), but differs greatly in solidity. It has 
some resemblance to the lateralist (Say), differing, how- 
*It is with some hesitation that I give this locality. The specimens are 
in my cabinet, labeled with this locality, but I cannot recollect on whose 
authority. 
| A recent species. Journal of the Academy of Natural Sciences, vol. 
2, p. 309 
