LAMELLIBRANCHIATA. 469 



Two subdivisions are comprised in this section. In the 

 first the siphons are short, and the pallial line is simple (Integro- 

 pallialia) ; as is seen in the families Chamidce, Hippuritidce, 

 Tridacnidce, Cardiadce, I/ucinidce, Cycladidce, and Cyprinidce. 



The second subdivision (Sinu-pallialia) is distinguished 

 by the possession of long respiratory siphons, and a sinuated 

 pallial line, and it comprises the families Veneridce, Mactridce, 

 Tcllinidce, Solenidce, Myacidce, Anatinidce, Gastrochcenidce, and 

 Pholadidce. 



Besides the impressions left by the muscular border of the 

 mantle, and by the retractor muscles of the siphons, when 

 these are present, there are other impressions caused by the 

 insertion into the shell of the muscles by which the valves 

 are brought together the " adductor muscles." The num- 

 ber of adductor muscles never exceeds two, but there may 

 be only one ; and in accordance with this distinction the 

 Bivalves have been divided into the two groups of the 

 Dimyaria and Monomyaria. These divisions, however, are 

 not of very high value. In most Bivalves there are two 

 adductor muscles passing between the inner surfaces of the 

 valves, one being placed anteriorly, in front of the mouth, 

 whilst the other is situated posteriorly, in the neighbourhood 

 of the vent. In the Monomyary Bivalves it is the posterior 

 adductor which remains, and the anterior adductor is absent. 

 The adductors leave distinct " muscular impressions," or scars, 

 in the interior of the shell, so that it is easy in any given 

 specimen to determine where there was only one adductor, or 

 whether two were present (see fig. 323). 



The habits of the Lamellibranchiata are very various. Some, 

 such as the Oyster (Ostrea), and the Scallop (Pecten), habit- 

 ually lie on one side, the lower valve being the deepest, and 

 the foot being wanting, or rudimentary. Others, such as the 

 Mussel (Mytilus), and the Pinna, are attached to some foreign 

 object by an apparatus of threads, which is called the " bys- 

 sus," and is secreted by a special gland, and for the passage 

 of which a special notch or aperture may exist. Others are 

 fixed to some solid body by the substance of one of the valves. 

 Many, such as the Myas, spend their existence sunk in the 

 sand of the sea-shore or in the mud of estuaries. Others, as 



