GENUS \VI. MYTILUS. 85 



The third division is distinguished by being " a little swell- 

 ing-" or sonicAvhat convex. These are of various shapes : some 

 species are nearly cylindrical, as M. Lithopliagiis ; many are 

 similar in form to the common Muscle ; some few have the 

 beaks placed at a little distance from the extremity, as in M. 

 ]\Iodiohis ; several species are nearly oval, with the beak and 

 hinge about the middle of the long side, as in M. cygneus and 

 anatinus. In this division the muscular impressions can but 

 rarely be perceived on the inner surface of the shells, though in 

 most of these the animal has four muscles, one of which is 

 much larger than the others : it protrudes no tube, but only 

 a strap-shaped foot, which it can extend to a considerable di- 

 stance from the shell, and with which it forms the threads of the 

 byssus (see Spectacle de la Nature, vol. 1. p. 137). In M. Hi- 

 rundo the shell is extended on each side the beak ; these parts 

 are called wings ; those which are on one side the beaks are 

 nmch larger than those which are on the other ; these have 

 but one muscular impression, and that is scarcely to be per- 

 ceived, as in the Scallops. The animal is nearly like that 

 of M. margaritiferus or Ostrea Malleus, fixing itself by a 

 byssus. The animal of M. Lithophagus has no byssus, neither 

 has the animal of M. stagnalis, cygneus, and anatinus ; they 

 project a scythe-shaped foot, and are nearly like the animal of 

 My a Picto7-um and margaritifera. 



The animals of the first division. Crista galli, Hyotis, and 

 Frons, are believed to be similar to the Tetliys of Linnaeus, or 

 the common Oyster. 



