. Bulletin of the Department of Geology. Geology. 408 University of California Publications. [Geology which are generally closely aggregated and nearly circular in cross-section. In some instances they are closely grouped and become angulated where they are in contact. The pillars are generally arranged in pairs set transverse to the longest diameter of the crushing face. In some cases three pillars are present in the transverse row. The average tooth comprises three pairs of pillars. In one specimen four transverse rows are present. In section (figs 15 and 3) the pillars are seen to consist o
. Bulletin of the Department of Geology. Geology. 408 University of California Publications. [Geology which are generally closely aggregated and nearly circular in cross-section. In some instances they are closely grouped and become angulated where they are in contact. The pillars are generally arranged in pairs set transverse to the longest diameter of the crushing face. In some cases three pillars are present in the transverse row. The average tooth comprises three pairs of pillars. In one specimen four transverse rows are present. In section (figs 15 and 3) the pillars are seen to consist of an ex- traordinary thick enamel layer and a com- paratively small dentine body. The enamel resembles in general characters that of the teeth in members of the mastodon group. In wear the pillars usually show a very thick rim of enamel surrounding a small central pit worn into the softer dentine. With wear the size of the central dentine area increases until, in a half-worn tooth, its diameter may about equal the thickness of the enamel ring. In the field the teeth are most commonly found broken up into fragments of pillars. Of the known Japanese specimens very fortunately one includes a large part of a skull with a number of teeth in the jaws. In this individual there are three cheek-teeth shown in the upper jaws (figs. 4 and Jl) and three in the lower (figs. 5 to 7). The anterior tooth in each jaw is much smaller than the tooth immediately behind it, and was considered by Yoshiwara and Iwasaki as P4 in the upper jaw and P:1 in the lower jaw. The anterior cheek-tooth, P*, of the upper jaw con- sists of four pillars of which the posterior pair are relatively quite small. The second upper cheek-tooth, M1, is at least three times as large as P4. It consists of eight large pillars, of which three form the anterior transverse row, two pairs form the second and third transverse rows, and a single pillar forms the posterior. Fig. 3. Desmostylus, sp. Fragment of a cheek-tooth showing
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