44 CONTRIBUTIONS 
In dedicating this species to my friend John Edward 
Grey, I must take occasion to make my acknowledge- 
ments for the advantages derived from the numerous and 
excellent memoirs which have come from his pen, and 
for many personal services while in Europe. 
M. pygmea. Plate l. Fig. 11. 
Description. Shell triangular, very thin, nearly equilate- 
ral, somewhat inflated, more produced before than behind, 
very indistinctly striate, shining; beaks rather elevated, 
pointed ; cardinal teeth large ; lateral teeth small; cavity 
of the beaks wide and deep. 
Diam. 3-20ths, Length 4-20ths,, | Breadth 7-20ths, of an inch. 
Observations. This small species is remarkable for being 
more produced before than behind. In this respect it differs 
from the lateralis of Say, to which it has some resemblance, 
though smaller. Immediately at the angle of the cardinal 
tooth, and below the point of the beak, there is a small 
apophysis, which I have not observed in other species. 
In the specimens of my cabinet, the muscular impress- 
ions cannot be perceived with a common microscope. 
The genus Mactra does not seem to have been noticed 
in England below the Crag. M. Al. Brogniart (Terrains 
du Vicentin) describes Mactra? erebea and Mactra? sirena 
from the calearéo-trappeens of the Val-Ronca. This I 
believe belongs to his Terrain Thelassique, equivalent to the 
Tertiary. He has, however, some doubts of their being 
