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.pygmaxi.. Plate 1. 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 calcareo-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 



