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. Linnceus. 



M. clathrodon. Plate 6. Fig. 223. 



Description. Shell subtriangular, thin, inequilateral, 

 obscurely and transversely striate ; beaks somewhat point- 

 ed ; lateral teeth crossed by equidistant minute strise ; 

 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 lateralis^ (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. 



t A recent species. Journal of the Academy of Natural Sciences, vol. 

 2, p. 309. 



