114 INVERTEBRATA OF MASSACHUSETTS. 



leaks not prominent ; epidermis pitchy-black ; within hluish- 

 ivhite ; teeth erect, conical, grooved. 



Figure 75. 



State Coll., No. 172. Soc. Cab., No. 2175. 



Alasmodonta arcuata, Barnes ; SlUimans Journ., vi. "277, pi. 12, f. 20. 



Mya margaritifera ? Lin.; Dillwyn ; Wood; «S:c. 



Margaritana margaritifera, Lea; Trans Amer. Ph'dos. Soc, vi. 135. 



Shell transversely much elongated, ovate or kidney-shaped, 

 thick and strong ; beaks within the anterior fourth, scarcely rising 

 above the line of the hinge, very much eroded ; hinge and basal 

 margins usually curved, nearly parallel ; nearly as broad before as 

 behind the hinge, and rounded ; more pointed behind, and the tip 

 appears as if slightly truncated ; surface somewhat waved by the 

 lines of growth ; epidermis close and smooth upon the disk, 

 loosely wrinkled towards the margin and posteriorly, color pitchy- 

 black. Within smooth, bluish-white, and sometimes tinted flesh- 

 color at the centre ; nacre not extending to the margin, leaving a 

 greenish border. Cardinal teeth two in the left valve, erect, strong, 

 pyramidal, the posterior one deeply grooved in front, so as to 

 form four or five denticles along its edge ; one on the right valve, 

 long, erect, a little twisted, deeply grooved along its front, and 

 with a pit each side, at base ; cavity of the beaks shallow. 

 Length 4| inches, height 2 inches, breadth 1} inch. 



Found in most running streams in the interior ; I have never 

 found it near the sea-board. 



It is a very common shell, and is at once known by its curved form, 

 dark color, and the want of a lateral tooth. It is the largest fresh- 

 water mussel we have. 



Mr. Lea regards our shell as identical with the European Mya mar- 

 garitifera of the older authors, the U'nio elongtda of Lamarck, &c ; 

 but the shells which I have had an opportunity of examining present 

 some constant differences. The foreign shell is shorter, the beaks 

 more nearly central and more elevated, and the portion of the in- 

 terior, within the palleal impression, is minutely granulated ; and, as 

 my foreign specimens agree accurately with the figures of Chemnitz 

 and Turton, I am induced to think there may be a constant difference. 

 The intervention of an ocean would strengthen the supposition. I 



