194 MYTILIDiE. 



at the extremity. Termination of the posterior side, which 

 is hut little dilated, rounded, but not broadly so. Urn- 

 bones prominent, not wide ; beaks acute, leaning forward, 

 slightly subspiral, not preceded by any lunule-like dulness. 

 Interior subnacreous white, not stained with coloured 

 markings ; hinge-margin a little reflected in front, not cre- 

 nated or denticulated, in aged shells subemarginated above 

 immediately under the beaks. Length of a rather large 

 individual an inch and three-eighths ; breadth scarcely 

 three-quarters of an inch. 



We have received the same shell, with the locality 

 " Georgia, United States/ 1 attached, from one of our Ameri- 

 can correspondents, as the M. castanea of Say (Journ. 

 Acad. Nat. Sciences of Philadelphia, vol. ii. pt. ii. August 

 1822, p. 266,) with whose language it is not at variance. 



CRENELLA. Brown. 



Shell equivalve, very inequilateral, tumid or compressed ; 

 surface covered with an epidermis, and either entirely or 

 partially ornamented by stria?, radiating usually in two 

 diverging fasciculi from the beak. Hinge margin toothless, 

 generally crenulated ; ligament linear, internal. Two 

 unequal muscular scars, pallia! impression obscure. 



Animal oblong, its mantle closed anteriorly, open in 

 front and in the branchial regions where the margins, 

 though not united to form a siphon, constitute pouting 

 and more or less puckered branchial lips ; formed into a 

 true and produced tube anally. Adductor muscles un- 

 equal. Foot narrow, ligulate, furnished with a byssal 

 groove. 



This group of Modiola, as they are usually considered, 

 has a fair claim to generic rank, equivalent to that of 



