IDENTIFICATION. 75 



Yet one more to be introduced to by name. The shell is 

 decidedly equivalve, but outrageously equilateral. It has no ears 

 and only a sort of apology for a tooth. Its ligament is internal, so 

 that if it is in the A division at all it must be in the last group. It 

 has no radial ribs, and is, consequently, neither Crenella nor 

 Modiolaria, but it has a pointed terminal beak, and the shelf inside 

 under the beak distinguishes it as Dreissensia, the fresh water mussel, 

 its neighbour in this tabulation, Mytilus, the salt water mussel, 

 having no shelf. As it is represented by only one species we have 

 no further to go. 



Let us have an instance which will take us beyond Division A. 

 Here is a fine, large, well-shaped shell with a splendid hinge that 

 ought to locate it at once, but does not. The shell is equivalve, 

 without ears, with few teeth, inequilateral, and with ligament 

 external; that is clearly neither an Anodonta nor a Unio. What is 

 it ? Let us go on to Division B. Its grooves are concentric, its 

 beaks are not twisted, its ligament is external, as we have seen, and 

 it is conspicuous and is T partially overlapped in a groove. The anterior 

 muscle scar does not project within the pallial line ; and thus we are 

 left at Cyprina, the only British representative of the Cyprinida, 

 which has only one species, islandica. 



And now let us deal with a specimen the other way round. 

 Here is another shell. It has been brought into us with the animal 

 in it. Being a bivalve we at once refer to the Pelecypoda to see 

 to what order it belongs. A rough dissection shows us that the 

 gill filaments are not divergent, that they are not parallel, that they 

 are not loosely connected, but that they are bound into plates. This 

 is enough to show that our specimen is one of the Eulamellibranchiata, 

 and we have then to place it in its family. 



It is unfortunate that there should be so many families in the 

 order, but after reading the notes as to some of the likely ones we 

 try the last in alphabetical order and find that it answers fairly well. 

 Its shell is regular and solid, its hinge has three large diverging 

 cardinal teeth, its ligament is external, its adductor scars are oval 

 and clear, and the pallial line has a particularly bold indentation. 

 Evidently we have one of the Veneridae to deal with. 



But the Veneridae are represented in British waters by five 

 genera; to which of them is it assignable ? To begin with, it has 

 unmistakably three cardinal teeth in each valve. This relieves us 

 of Lucinopsis, which has only two cardinals in the left valve. Neither 

 of the six teeth is distinctly cloven, though there is just a trace of a 

 beginning of division. This allows us to pass Circe, which has the 

 posterior cardinal in the left valve cut in two. It has no well-marked 

 concentric plates, only concentric ridges, and so it cannot be 

 Venerupis, thus it must either be Venus or Tapes, and as Venus has 

 an angular sinus in the pallial line, while this specimen has a 

 distinctly marked line with a deep well-rounded bay, we have no 

 difficulty in declaring its genus to be Tapes. 



We may as well find its species. It is concentrically striated, 

 and consequently can be neither decussatus, in which the striations 

 are radial and strong, nor pullastra, in which the striations are 



