264 MARINE SHELLS 



M. demissus and exustus ; from the former it is 

 distinguished, by not bavins; the angle on the po>te- 

 rior side obtusely rounded, and not placed consider- 

 ably before the middle ; and the line of the edge be- 

 fore this angle, is not convex as in that shell. It 

 does not at all correspond with the figures in the 

 Encyc. Method, which are quoted for exustus ; but 

 it agrees very well, and is probably specifically the 

 same, with the species represented on plate 365 of 

 Lister's conch, which the author thus defines (i mus- 

 cutus parvus, subluteus, leviter striatus." 



2. M. ^lateralis. Shell transversely suboval, 

 inflated, subpellucid, with numerous concentric 

 wrinkles, anterior and posterior margins, longitudi- 

 nally ribbed with alternate large and small lines, 

 which crenate the basal margin ; intermediate area 

 destitute of longitudinal lines ; most prominent part 

 of the shell extending from the beak to the tip of the 

 anterior margin, and bounded on its posterior side by 

 an indented line ; epidermis pale-brownisb. 



Length three tenths of an inch. 



Breadth eleven-twentieths of an inch. 



Thickness seven- twentieths of an inch. 



Inhabits the southern coast. 



Cabinet of the Academy and Philadelphia Mu- 

 seum. 



Found imbedded in the large Tethya of our coast. 

 This shell is closely allied to Mytillus discors of 

 Montague. 



