VARIETIES OF OYSTERS. 157 



attention of naturalists to the elucidation of this simple 

 point. On it depends the Lamarckian division of the 

 Lamellibranchiata into Monomyaria and Dimvaria, the 



J * 



oyster being the type of the former group. 



Dr. Fischer says that the adductor muscle in Pecten 

 (which is allied to the oyster, and belongs to the same 

 group,) is divided, so as to form anterior and posterior 

 bundles placed at different angles. He is of opinion that 

 the group of Monomyaria exists only in appearance, and 

 not in reality. The cartilage and ligament advance with 

 the growth of the animal, in consequence of which the old 

 layers become useless and are external. The oldest or 

 first-formed portions of the shell cease in time to be occu- 

 pied by the animal, so that the beaks become disunited, 

 and in adult specimens are separated by a wide chasm. 

 The shell is remarkably calcareous, and consists for the 

 most part of layers termed by Dr. Carpenter " sub- 

 nacreous," and having comparatively little adhesion one to 

 another. These layers are internal. The outer layers are 

 composed of prismatic cellular structure, and have no 

 natural cohesion. The weight of the animal in a full- 

 grown example is very disproportionate to that of the 

 shell. 



The late Mr. Thompson, of Belfast, ascertained that 

 a large oyster from that bay weighed altogether two pounds, 

 but that the weight of the animal taken out of the shell was 

 onlv an ounce and a half. 



j 



Large-sized specimens from the British seas seldom 

 exceed six inches in length ; but on the North American 

 coast this species (if it be the same as ours) is said to 

 attain occasionally twice that size. Young shells are 

 sometimes marked with radiating purple streaks, and now 



