. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. WOOD AND PATTERSON : OLIGOCENE RODENTS OF PATAGONIA 341 3 or 4, and M2 wear stage 2 or 3. and yovy shortly after has come into use. The crowns of all the teeth were covered b}' a complete enamel cap, even though it was removed very quickly by wear. That on the upper surface of the talonid of P4 was so thin as to appear absent (Fig. 19E). Its thinness in this specimen cannot be due to wear, however, unless the wear was against the roots of dm4, since we personally removed the roots from over this part of the crown
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. WOOD AND PATTERSON : OLIGOCENE RODENTS OF PATAGONIA 341 3 or 4, and M2 wear stage 2 or 3. and yovy shortly after has come into use. The crowns of all the teeth were covered b}' a complete enamel cap, even though it was removed very quickly by wear. That on the upper surface of the talonid of P4 was so thin as to appear absent (Fig. 19E). Its thinness in this specimen cannot be due to wear, however, unless the wear was against the roots of dm4, since we personally removed the roots from over this part of the crown in no. 3113. The enamel extends to varying distances down the sides of different parts of the cro^vn of the cheek teeth, due to the asymmetric development of hypsodonty. It is interrupted, after the tooth is about half worn down, along the anterior faces of. A B C Fig. 20. CepJialomys arcidens Ameghino, lower teeth, x 5. A, Rdm^, no. 3013; B, Ldm4, no. 3011; C, cross-section of EIi, anterior view, no. 3155. the lower molars (Figs. 19D, F). Before this level is reached, the thickness of the enamel is considerably reduced by inter- dental wear (Fig. 19A). The enamel is next lost on the posterointernal corner of the entoconid, beginning with Mi. This occurs much earlier in C. plexus (Fig. 19F) than in C. arcidens. A layer of cement is deposited around the basal portions of the crown, particularly in C. plexus, which sometimes makes it difficult to determine the exact points at which the interruptions occur. In the upper molars, since the teeth are high-crowned lingually and low- crowned buccally, the initial point of loss of enamel is the buccal surface of M^, followed in turn by M- and M-^. The enamel is then lost on the posterior faces of the same teeth. Before the level of no enamel is reached, the buccal enamel becomes con- siderably thinned, which is obviously not due to interdental Please note that these images are extracted from scanned p
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