The Pectinibranchiata comprise all the spiral univalve shells, and are by far the most numerous of all the gasteropods, as their species are not counted by hundreds, but by thousands. If their calcareous garment could be drawn out, it would be found to consist of a tube gradually widening from the apex to the base; but what an immense variety of form and ornaments, what a prodigality of splendid tints, has not Nature spread over this interminable host! The same fundamental idea appears to us in thousands of modifications, one yet more elegant and capricious than the other. Thus the passion of the shell collector is as conceivable as that of the lover of choice flowers, and when we read that rich tulip-amateurs have given thousands of florins for one single bulb, we cannot wonder that many of the Volutes, Cones, Mitres, and Harps, are worth several times their weight in gold; that more than a hundred pounds have been paid for a Chinese wentle-trap, and that the Cypræa aurora, which the Polynesian chiefs used to wear about the neck, is valued at thirty or forty guineas.
Orange Cone-Shell.
Mitre-Shells.
Harp-shell.
The mode in which these beautifully painted structures are formed is very similar to what takes place among bivalve shells. They are secreted by the glandular margin of the mantle or soft skin which clothes the upper part of the body of the snail, and their form depends on the shape of the body they are destined to cover, while the outline of the border is alike regulated by that of the mantle. In the border of the mantle are placed the glands through which colouring matter is added to the lime of which the shell consists, and here also the whole of the outer coat of the shell is formed by constant annual additions to the lip. The after-growth of the shell proceeds, layer over layer, from the general surface of the mantle, so that the calcareous robe constantly increases in thickness with the age of the animal.
Chinese Wentle-trap.—(Scalaria pretiosa.)