PROC. BIOL. SOC. WASH. 102(1), 1989, pp. 3-7 A NEW GENERIC NAME FOR TATE'S (1933) MICROTARSUS GROUP OF SOUTH AMERICAN MOUSE OPOSSUMS (MARSUPIALIA: DIDELPHIDAE) Alfred L. Gardner and G. Ken Creighton Abstract. —Until recently, all mouse opossums were considered as belonging to the genus Marmosa, as revised by Tate (1 933). Today, however, we recognize that Marmosa (s.l.) contains several distinctive species groups whose relation-ships are best expressed by dividing this complex into five genera as follows: Marmosa (s.s.), Marmosops, Micoureus, Thylamys, and a complex of species for which we propose a new name Gracilinanus because no genus-group name is available. Systematists have differed in their inter-pretation of the generic and subgeneric tax-onomy of the smaller mouse-sized marsu-pials of the family Didelphidae. Thomas (1888) grouped the known taxa of mouse opossums under the subgenus Micoureus in the genus Didelphys. Matschie (1916) dis-tributed these taxa among five subgenera of Didelphis: Marmosa, Grymaeomys, Mar-mosops, Thylamys, and Caluromys. Ca-brera (1919) grouped mouse opossums into two subgenera {Marmosa and Thylamys) under the genus Marmosa. Tate's (1933) re-vision was the most comprehensive. Tate (1933:22) believed that the genus Marmosa was a "'good,' natural genus of didelphids" that included all Neotropical mouse opos-sums. Although Tate acknowledged natural subunits in his concept of Marmosa, he did not recognize subgenera and, instead, used the following names to indicate five infor-mal groups: cinerea, murina, noctivaga, mi-crotarsus, and elegans. He also expressed the opinion (p. 22) that "there is always the chance that subgenera may be later con-verted into full genera, and thus, in the case of Marmosa the undoubted unity of the ge-nus be obscured." Over the ensuing half-century, however, new information has made the division of Marmosa (sensu Tate 1933) into several genera a desirable and necessary step to bet-ter reflect relationships and natural assem-blages. Gilmore (1941) again divided Mar-mosa into the subgenera Marmosa and Thylamys but did not list species. Cabrera (1958) used Marmosa and Thylamys as sub-genera as he had earlier (Cabrera 1919) and followed the species associations of Tate (1933) except that he divided Tate's micro-tarsus group, assigning the microtarsus sec-tion to Thylamys and the lepida section to Marmosa. Marshall (1981) listed the names Marmosa ("murine opossums"), Micoureus ("large murine opossums"), and Thylamys ("small murine opossums"), as genera among other didelphids and attributed this arrangement to a manuscript by Reig et al. subsequently published in 1985. Reig et al. (1985) assigned Tate's (1933) murina and noctivaga groups to the genus Marmosa along with the more recently described taxa M. andersoni, M. cracens (incorrectly spelled crascens), and M. xerophila. They allocated Tate's (1933) cinerea group to Micoureus. They also assigned the 14 species listed by Kirsch & Calaby (1977) along with Mar-mosa lepida, M. emiliae, and M. contrerasi, to Thylamys. Creighton (1984) also recog-nized five groups in his revision of the genus
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