1(39 



from the South Seas, which were an inch and a half in 

 breadth; whereas the BritiJIi fpecimens rarely exceed 

 half an inch in breadth, and three-eighths of an inch in. 

 length. 



This fhell has frequently been confounded with the fol- 

 lowing fpecies, from which it is perfeftly difiinft, as may 

 be feen by the comparative defcription there given. 



11. 

 M. with a fub-oval, compreffed, pellucid, thin, brittle Disgrepans. 

 fhell, covered with an olive-green epidermis: and, like 

 the preceding fpecies, is divided into three compartments, 

 the poderior and anterior fides ribbed, or ftriated longitu- 

 dinally ; the middle area (lightly wrinkled : umbo placed 

 as the laft, but very little prominent : both fides rounded, 

 the anterior one largeft. Infide white, with a faint pearly 

 glofs. Inferior in fize to the preceding. 



The fingular flrufture of this and the M. difcors, fo 

 much refemble each other, at firft fight, that the eflential 

 fpecific cha rafters have been overlooked : and we once re- 

 ceived a fpecimen oFeach from a friend, an able concho- 

 logift, for the fame fhell. 



The principal diftinftions are, that this is very little 

 convex, whereas the other is nearly as thick as it is broad ; 

 this is rounded, and liroidefl at the anterior fide; the other 

 is fmalleff on that fide, and runs out to an obtufe point : 



Z it 



