. Bulletin - United States National Museum. Science. NEW iSPEICaiES OiF EXTTN-CT MTOCENB SIREiNIA 81 The upper and lower molars have few trenchant differ- ences, usually the anterior lake of the posterior upper molars is rather large and the main cusps are well developed. The anterior lake of the posterior lower molars is reduced and the main cusps are less prominent. Regressive processes associated with the erupting per- manent or succeeding cheek tooth resulted in the entire absorption of each milk tooth except the hollow cuplike enamel crown. The enamel crowns (pi. 35, figs, 5, 6) of the tw
. Bulletin - United States National Museum. Science. NEW iSPEICaiES OiF EXTTN-CT MTOCENB SIREiNIA 81 The upper and lower molars have few trenchant differ- ences, usually the anterior lake of the posterior upper molars is rather large and the main cusps are well developed. The anterior lake of the posterior lower molars is reduced and the main cusps are less prominent. Regressive processes associated with the erupting per- manent or succeeding cheek tooth resulted in the entire absorption of each milk tooth except the hollow cuplike enamel crown. The enamel crowns (pi. 35, figs, 5, 6) of the two opposite deciduous cheek teeth each measure 18 mm. in length and 13 mm. in width. They are here regarded as the third lower deciduous cheek teeth. For these milk teeth (USNM 16630) the terminology of the permanent molar is applied topographically in view of the uncertain homologies of the cusps on the crowns of the deciduous cheek teeth. Me'. En* Figure 36.—Posteriormost lower cheek tooth, right mandible, USNM 16630, of Metaxytherium calvertense. Abbrs.: En"*, entoconid; H/i, hypoconid; Hyc"*, hypoconulid; Me*, metaconid; Pr*, protoconid. The buccal protoconid and the opposite metaconid are both blvintly conical and are partially separated by a shallow narrow cleft. The small anteromedian cuspule is pressed against the protoconid and also partially ob- structs the deep transverse valley centrally. The entoconid is slightly smaller than the hypoconid; these two conids are so closely approximated that they tend to conceal the posteromedian cuspule. The large buccal hypoconulid is cut off from the hypoconid by a deep cleft. The posterior talon located on the lingual side of the hypoconulid is noticeably cuspidate. No plausible explanation has been advanced to support the contention of Lepsius (1882, p. 106) that sirenian cheek teeth increase in size during advancing age. It is generally accepted that once the enamel crown is fully formed any size increase of cheek te
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