HANDBOOK OF SHELLS. 



centre ; this is the point to which 

 the muscles that close the valves 

 and hold them so tightly together 

 are attached. In the majority of 

 bivalves, however, there are two 

 such muscular impressions, or scars, 

 one on either side of each valve 

 of the shell. The former group 

 on this account are often called 

 Monomyaria (having one shell- 

 muscle), and the latter Dimyaria 

 (having two shell-muscles). In the 

 last named the two muscular im- 

 pressions are united by a fine groove 

 (or pallial-line), which in some runs parallel to the margin of 

 the shell (Fig. 15), whilst in others it makes a bend in (pallial- 

 sinus) on one side of the valve towards the centre (Fig. 16). 

 In Monomyaria it will be found running parallel to the margin 

 of the shell. It marks the line of attachment of the mantle or 

 shell-secreting organ 

 of the animal to the 

 shell which grows by 

 the addition of fresh 

 matter along its edges, 

 so that the concentric 

 curved markings so 

 often seen on the 

 exterior correspond 

 in their origin with 

 theperiodic mouths of 

 the Gasteropods. The 

 bivalves are all 

 aquatic, and many 

 bury themselves in 



the sand or mud by Ven us plicata. 



means of a fleshy, 



muscular foot. These are furnished with two siphons, or fleshy 

 tubes, sometimes united, sometimes separate, through which 

 they respire, drawing the water in through one and expelling it 

 by the other. Those kinds whose habit it is to bury themselves 

 below the surface of the mud or sand are furnished with long 

 retractile siphons, and to admit of their withdrawal into the 

 shell, the mantle is at this point attached farther back, giving 

 rise to the pallial-sinus above described ; this sinus is deeper as 



