MACTRACEA. MOLLUSCA. MACTRA. 55 



Shell small, triangular, tumid, nearly smooth, shining ; nearly- 

 equilateral, the posterior side somewhat prolonged, and sloping 

 less rapidly than the anterior side ; ends rounded ; beaks elevated, 

 not meeting, pointed and inclined forwards ; the regions before 

 and behind the beaks are broad, flattened, and more or less heart- 

 shaped, defined by slightly elevated ridges ; surface finely marked 

 by the lines of growth, white, covered with a thin, dirty-brown epi- 

 dermis ; hinge strong, the pit for the cartilage being a small recess 

 penetrating deeply into the beaks ; before it is a strong, promi- 

 nent V tooth, and on each side of it, in the left valve is a stout 

 and prominent lateral tooth, and in the other a deep fossa with 

 elevated sides to receive it ; cavity of the beaks deep ; muscular 

 impressions deep ; palleal impressions distinct, with a shallow 

 sinus posteriorly ; interior clear glossy-white. Length | inch, 

 height /jj- inch, breadth -p\ inch. 



The only places, where I have found this shell living, are the 

 inlets of the salt marshes between Roxbury and Boston. But all 

 the flats, which have been drained by the erection of the Mill dam, 

 have a layer of them just beneath the surface ; and vast numbers 

 were un-earthed in throwing up the embankments for the rail-ways 

 which cross them. They doubtless exist plentifully in the bed of 

 the river on the other side of the Mill-dam. They are found 

 abundantly at New Bedford also, and I know them to be common 

 about Rhode Island. 



It assumes very various forms, depending mostly on age. When 

 young, the shell is thin, rather compressed, and the beaks are incon- 

 spicuous and touching each other. By age it becomes very thick and 

 turgid, the beaks elevated and widely separated, and the height of the 

 shell often equals its length. 



The small, deeply penetrating pit of the hinge is very peculiar. 



The dimensions of one from the track of the Providence Rail- 

 road is as follows. Length -^^ inch, height -^^ inch, breadth -^^ 

 inch. It is represented by Figure 34. 



It is not likely to be mistaken for any other species. The young of 

 M. solidissima and M. ovalis are much less triangular, and of a very 

 different aspect. It has more the proportions of M. solida of Europe. 



