Yet of all the life histories revealed by the pond at low tide I still think that of the _Unionidæ_ the most interesting. You find them all along above and below the margin of the shallow water, their shells most wonderfully streaked with olive-green and pale-yellow in alternate bands, till one might think he had found nodules of malachite which the long-ago glacier had culled from some Labrador ledge and ground to unsymmetrical ovoids before it dropped them on the old-time meadow marge. In certain individuals and certain lights the shells of these obscure creatures send out gleams of green and gold, like gems that have soft fires within them. It is as if an opalescent soul dwelt within, and the thin shell which a crow with his bill may puncture with a blow was so constructed as to hold in the reds and blues of the opalescence, but transmit the greens and gold. You find many with only the backs of their shells sticking out of the mud. This may be the creature’s natural position, but I find far more of them lying quietly on their sides in the shallow water, rocking gently to and fro in the placid undulations as if they were there but to show me their shining colors. But if you watch one intently for a time you will see him open his shell cautiously and put out one foot. This is his best, for it is all he has and he puts it foremost. It is very white and clean, and it might as well be called his tongue, for with it he licks his food. It is half as long as he is, and when he has put it out as far as he can, or as far as he dares, a fine white fringe grows on its outer margin. Thus he gathers in minute animalculæ or refuse matter from the surface of the mud, for his stomach’s sake. It is a rather interesting thing to stand by and watch a _Unio margaritifera_ daintily putting away his own particular brand of little necks and mock turtle. At the least untoward sign of interest in the affair, however, he shuts up like a clam, and you will need your pocket-knife if you wish to see more of him. Where the water is only an inch deep or so over the soft ooze of the bottom you will see where the unio has used this so-called foot as a foot should be used, for he not only stands on it, but walks with its help. These signs are curiously erratic marks drawn as with a sharpened stick for a distance sometimes of yards. If you will inspect the seaward end of this trail you will find a unio in it, generally a young one, for it is he that has left the mark behind him in his travels. For the unio at a certain age is a great traveller; that is, when he is very young. The adults foot it, but the young before they reach their full growth ride, some of them by what you might call the lightning expresses of the pond world.