34 MULTIVALVES AND BIVALVES. 



The bivalves used to be classified on the pallial line ; it was not 

 a perfect system in some ways, but it comes in excellently as a 

 guide in dealing with the shells only, and, with a few qualifications 

 of the free and easy sort, we will adopt it here. It is possible to 

 divide the group into those with the pallial line indented and those 

 with it entire, but those without the indentation are so numerous 

 that the table is too long to work with readily. Hence we will keep 

 to the old lines, and, as shown in the alphabetical list of British 

 genera herewith, have three categories: 



A. Those having the pallial line not indented and having 



no siphons. 



B. Those having the pallial line not indented and having 



short siphons. 



C. Those having the pallial line indented and having long 



siphons. 



In the C division everything is straightforward. With regard to 

 A, all that can be done is to run through it, and if the genus cannot 

 be discovered, to pass on to B, which is not so difficult after all, and 

 will be found to be much more exp'aditious than with any com- 

 bination of the two. We say genus, for the British bivalves are com- 

 paratively so few that we can afford to disregard orders, sub-orders, 

 and families, and deal with the genera direct, which is more satis- 

 factory, as they are least likely to be affected by changes in 

 classification. 



Let us look, then, if our shell's pallial line be indented or not, and 

 if it be, we will assume that the animal had no siphons. We will 

 then ask if the valves are of the same size, or is one larger than the 

 other? Is it 



1. Inequivalve, or 



2. Equivalve ? 



Supposing it be inequivalve, is it equilateral, or nearly so ? That is, 

 does a straight line dropped perpendicularly from the beak to the 

 ventral margin divide the surface of the valve into halves ? If it 

 does, has the shell any ears that is, winged processes on the side 

 of the beak ? If it has ears, the shell is a Pecten ; if it has none, it 

 is either Anomia or Ostrea : the former if it has the hole in its lower 

 valve through which passes the byssus by which it is fastened to the 

 rock, the latter if it has no hole. Here we have: 



Equilateral, or nearly so 

 With ears Pecten. 

 Without ears 



With hole Anomia. 

 Without hole Ostrea. 



If the shell is markedly inequilateral, we can further advance to : 



Very oblique 



With teeth Avlcula. 

 Without teeth Pinna. 



Neither of these is like anything else : in fact, the five genera thus 

 chosen are easy ones to begin with. 



