Mussel, from the peculiar shape of its shell, offers an exceptional case; and I am inclined to think that in the case of bivalves of a more flattened shape, the earlier statement holds true. At South Shields, England, perforated half-shells of the Common _Venus_ (Fig. 34) are so abundant that the children string them for necklaces; yet I have never been able, by the most industrious search, to find more than one or two specimens in which the hole is at all near the lip of the shell. It is possible that these exceptional instances were the work of a young and inexperienced univalve mollusc, or a stupid one. It is possible, also, that the mode of attack differs somewhat according to the species of the assailant. (It should perhaps be explained, for the benefit of those who have no experience in the ways of children or of shell necklaces, that the hole must be moderately near the beak of the shell, to enable the shell to "sit" properly on a string. Every unit in the necklace may therefore be counted as one in favour of the older theory.) Many of the Siphonostomatous molluscs are surprisingly active and strong, so that they are well fitted for a predatory existence. In fact, they not only eat bivalves, but occasionally attack the vegetable-feeding univalves when nothing better is to be got, so that occasionally the shells of these also may be found displaying the deadly little round hole we have described. [Illustration: FIG. 34.--Half Shells of the Common Venus, several of them perforated by carnivorous molluscs. From South Shields, England.] Let us contrast with the _Murex_ one of the shells which are "holostomatous," _i.e._ possessing an unindented shell-mouth--that is to say, one without a "siphon." The common edible periwinkle, _Littorina littorea_, may be taken as an example. No shell is more familiar; even the town-dweller, who has never found it on the sea-shore, has seen it often on stalls in the slums. The mouth of the shell is quite round and unindented, and in this case the character holds good as the mark of a vegetable-feeder--a non-predatory sea-snail. It is hardly necessary to remind the reader that its name (the shore-shell) is given it because it lives where the tide leaves the rocks exposed during part of the day. Another common species of _Littorina_, which frequently lives a little lower down, where the large sea-weeds grow, has been described in Chapter II.; and another, _L. rudis_, lives a little higher up, so that it spends most of its time in a dry state, and is fast on its way to become a land-shell. At most of the familiar English seaside resorts one may see dozens of it baking in a hot July sun on rocks where only the highest tides can reach them: and yet under these conditions they continue to live and flourish. The periwinkles are remarkable for the great length of the tooth-ribbon, in comparison with the size of the animal. The number of separate teeth upon it has been estimated at 3500.