3. The teeth of the walrus, or sea-cow.—­These are nearly straight, and measure from 2 feet to 2-1/2 feet in length.  The exterior portion of the tooth possesses a much finer grain and texture than its core, which in appearance and properties bears a close resemblance to ordinary bone.  Of a yellowish cream-colour and mottled, this ivory is much less valuable than the teeth of the hippopotamus.  It is seldom applied in our day to other than dental purposes; but its antiquity is interesting.  The Scandinavian relics of the eleventh and twelfth centuries, with which our museums are so profusely enriched, are for the most part formed of the teeth of the walrus.  The elegant spiral horn of the narwhal or sea-unicorn also produces ivory of a superior quality.  It is not to any great extent applied to useful purposes, but is more frequently preserved in museums and collections as a beautiful natural curiosity.

The tusks and teeth of the elephant—­the latter, for the sake of distinction, are termed grinders—­are formed after the ordinary manner of the teeth of animals.  The organism which converts the earthy constituents of the blood into cellular tissue and membrane, contributes in the same way to form the teeth by the successive deposition of layer upon layer of the soft vascular pulp.  The marks of these depositions, or laminae, are clearly distinguishable in the longitudinal striae of the section of a tooth.  Mr Corse Scott states that the Indian elephant has only ten or twelve laminae in the tooth, while that of the great mammoth has twenty-four, besides having a much more regularly disposed enamel.  The tooth is hollow about half-way up, but a very small tubular cavity is visible throughout its entire length.  This, sometimes called the nerve, is in reality the apex of successive formations in the process of growth.  The grinders are seldom used in the arts.  They are of a different texture, the laminae more loosely combined, and possessing a tendency to separate, which renders them unfit for nearly all useful purposes.  Ivory has the same chemical constitution as ordinary teeth—­that is, cartilage united to such earthy ingredients as the phosphate of lime.

But it is very remarkable that the fossil ivory of the mammoth, and specimens of the historic period of Pompeii or Egypt, contain sometimes as much as 10 per cent. more of fluoride of calcium than the ivory of the present day.  We apprehend, however, that this property—­first investigated by Dr George Wilson—­may be derived from long-continued contact with earth, since fluoride of calcium is the chief ingredient in the enamel or exterior portion of the tooth.  Ancient ivory, having thus gained in its inorganic bases, becomes deficient in the gelatinous constituents necessary to its preservation.  We recently had a singularly beautiful application of the knowledge of this principle in the case of the ivory specimens sent from Nineveh by Mr Layard.  On their arrival in England, it was discovered that they were rapidly crumbling to pieces.  Professor Owen recommended that the articles should be boiled in a solution of albumen, which was done accordingly, and the ivory rendered as firm and solid as when it was first entombed.