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The spire is a prominent feature of the Univalve; and upon its being elevated, depressed, &c., depends much of the generic and specific definition. Adanson, in his ‘Natural History of Senegal,’ says that the external character of the spire varies according to the plane they turn upon, which, he observes, is either horizontal, cylindrical, conic, or ovoid. At the same time, he admits that there are a great many intermediate forms which cannot properly be defined.

It must be remembered that many of the young shells have not the same number of wreaths as the adults; from which it would appear, that the part of the animal nearest the apex never increases in size. The number of wreaths cannot, at all times, be depended upon. A full grown shell may, however, be known from the outer lip, which has generally an unfinished appearance in young shells. Indeed, in all the land and fresh water shells it is a distinct criterion, as they are never complete in the form of the outer lip till full grown.