36 MULTIVALVES AND BIVALVES. 



Now let us deal with those in which the valves are of equal size. 

 These we can divide into those with ears and those without ; and 

 those with ears we can then subdivide into that with teeth, which is 

 Limopsis, and that without, which is Lima, the latter being further 

 distinguished by having one muscular scar while the former has two. 

 We thus get : 



Shell equivalve 

 With ears 



With teeth Limopsis. 

 Without teeth Lima. 



We can continue this : 



Without ears 



Hinge with many teeth 

 Hinge straight Area. 

 Hinge curved 



Shell round Pectunculus* 

 Shell elongated Leda. 

 Shell trigonal Nycula. 



Here Area, with its long, straight row of teeth all alike, is un- 

 mistakable ; Pectunculus has its teeth in two groups, one on each 

 side of the beak, and affords the first instance of the evolution of the 

 forms of teeth ; in Leda the shell is lengthened out at the posterior 

 end so as to give it a somewhat leg-of-mutton appearance, and the 

 teeth on each side of the beak are nearly equal in number ; in 

 Nucnla, which has a shell like a nut, the teeth are much more 

 numerous on the posterior side than on the other. 



The next group can include those having few or no teeth, and 

 one of them, Axinus, can be picked out at once, owing to its shell 

 being almost circular. That leaves us with the rest inequilateral, 

 which we can separate into those with the ligament external and 

 those with it internal. The former we can further sort into : 



Teeth conspicuous Unio. 

 Teeth inconspicuous Anodonta. 



Those with an internal ligament are only four in number : 



Crenella, which has radial ribs and one upright tooth, the hinge 

 margin being crenulaied behind the ligament. 



Modiolaria, which has two groups of striations, one down each 

 margin. 



Dreissensia (the fresh-water mussel), which is very oblique, and 

 has the beaks terminal and pointed and has a shelf within 

 the beak. 



Mytilus (the salt-water mussel), which is of similar shape but 

 has no shelf. 



If our specimen is not one of these, and they are all distinct in 

 form, we must proceed to the genera in which the pallial line is not 



