138 of narwhal tusk reveal indentations in the cementum and frequently display longitudinal cracks that follow the spiral pattern, making them unique among mammalian tusks. These cracks can penetrate through the dentin into the pulp cavity. Narwhal tusk has a clearly defi ned transition ring between the cementum and the dentin. The deposition of the dentin can also display prominent circular rings, and the pulp cavity appears as a large hollow center that extends throughout most of the length of the tusk (fi g. 4f). HIPPOPOTAMUS Hippopotamus amphibius The semi-aquatic hippopotamus’ domain is largely restricted to sub-Saharan Africa, although the species has also been known to occupy stretches of the Nile River and the Nile Delta. The World Conservation Union, the authority in species populations, recognizes the hippo as a vulnerable species due to unregulated poaching that has left the hippo population at a little over 100,000.25 At present, relatively little regulatory effort is being made to protect land mammals other than elephants, making the commercial value for ivories derived from hippos especially valuable on international markets. Most signifi cantly, the crackdown on elephant ivory has increased the value of hippo ivory as a substitute.26 Perhaps second to elephant ivory, hippo ivory is prominent in tribal art (fi g. 9), making its correct identifi cation especially important.27 Hippo ivory comes from both the upper and lower canine and incisor teeth, which grow continuously throughout the duration of the animal’s life. The incisors have been known to reach 40 cm in length and the canine teeth can grow up to 50 cm. They do not project beyond the lips, so they are correctly called teeth rather than tusks. The thin layers of enamel and cementum are usually removed when the teeth are processed and carved into art objects. The dentin is characterized by packed lines that follow the overall shape of the tooth. The curved upper canine has a deep longitudinal indentation that extends the entire length of the tooth on the inner (lingual) surface of the curve, and this is a prominent feature in cross-section analysis.28 The cross section of the lower canine appears triangular. Following the shape of both teeth, the pulp cavity appears as a curved or arched line (fi gs. 4g and h). Hippopotamus’ incisors are peg shaped, and cross sections reveal lines from cracks penetrating through the outer layer of circularly deposited dentin, although these seldom break through to the core of the tooth. The pulp cavity is also lined with circularly deposited secondary dentin with the center of the pulp cavity appearing hollow as a small dot or line and forming a relatively small surface area of the tooth (fi g. 4i). WARTHOG AND BOAR Phacochoerus aethiopicus and Sus scrofa The warthog faces few of the same threats as the other mammals in terms of endangerment. Currently, the International Union for Conservation of Nature lists the common warthog in the “least concerned” category.29 The warthog population is distributed widely across sub-Saharan Africa and has an estimated population within the range of the lower to mid 200,000s. Warthog ivory serves an important role in tribal art (fi g. 10) and is widely traded on international markets, although its appearance and relative abundance make it less valuable relative to elephant ivory.30 A typical warthog will have two sets of upper and lower canine tusks curving radically upward from the mouth. The upper canine teeth that comprise the larger of the two sets can grow FIG. 13 (right): Box. Yoruba, probably Owo, Nigeria. Before 1873. African elephant ivory. L: 19 cm. Donated by William Bragg, 1873. British Museum, London, inv. Af.8801.a–b. © The Trustees of the British Museum.
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