HALF AN HOUR WITH SHELL-FISH (BIVALVES). 209 



the rocky sea-bottom. In all bivalves there are 

 certain muscles called "adductor muscles," which 

 are under the control of the animal, and used for 

 closing the two shells together. How powerful 

 these are, everybody knows who has tried to open an 

 oyster the oysterman being obliged to cut them in 

 two with his sharp knife before he can open the 

 shells. Of course, these muscles require to be very 

 firmly attached to the interior of the shells, and 

 such is the case, for we find that in dead shells, after 

 the animal matter has been entirely removed, there 

 are left the distinct impressions of the muscular 

 scars. Formerly, bivalve shells were classed in two 

 groups, according as they had two of these scars, or 

 only one. It will be seen, therefore, that an atten- 

 tion to these scars, and their number and position, 

 affords one good means of identifying species. 

 Again, the margin of the mantle has also a muscular 

 property, and it frequently leaves its impression in 

 the interior of the shell as well, forming what is 

 called the " pallial " line. An attention to this line 

 is useful. In some bivalves we find it unbroken, 

 when we know that in them the mantle-lobes were 

 quite free. But, where the pallial line is indented, 

 this is taken as an indication that the mantle-lobes 

 were united to each other. It affords the knowledge, 

 at the same time, that the animal had siphons, 

 which could be withdrawn into the shell. This 

 class of bivalves generally live in sand or mud, in 

 which they bury themselves, leaving the siphons 



