SERIES PUBLICATIONS OF THE SMITHSONIAN INSTITUTION Emphasis upon publication as a means of "diffusing knowledge" was expressed by the first Secretary of the Smithsonian. In his formal plan for the Institution, Joseph Henry outlined a program that included the following statement: "It is proposed to publish a series of reports, giving an account of the new discoveries in science, and of the changes made from year to year in all branches of knowledge." This theme of basic research has been adhered to through the years by thousands of titles issued in series publications under the Smithsonian imprint, commencing with Smithsonian Contributions to Knowledge in 1848 and continuing with the following active series:

We provide an updated list of the Recent land mammals of Mexico and include information on the taxonomy of certain species, and where appropriate, the endemic and threatened status of all species listed. Several taxonomic and nomenclatural changes have been made since publication of the last list of the Mexican terrestrial mammalian fauna. Within the period from 2005 to present, there have been at least 209 changes concerning the nomenclature of this fauna; these we evaluated in this paper. The land mammals of Mexico comprise 168 genera, 496 species, and 881 subspecies.

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One hundred and seventy-seven specimens of American didelphids, representing 9 genera and 22 species have been studied for their chromosomal constitution. Didelphids are very conservative in chromosomal complements. All of the studied species can be sorted into one of three kinds of karyotypes: 2n = 14 (three species of Didelphis, one of Lutreolina, two of Philander, and one of Chironectes), 2n = 14 (eight species of Marmosa. one of Metachims, three of Caluromys, and one of Dromiciops), and 2n = 18 (three species of Monodelphis). These karyotypes are stable, showing only minor variations within each basic pattern. I t is concluded that chromosornals evolution in the Didelphidae proceededs from low numbers to higher numbers by a process of centromeric fissioning complemented by some pericentric inversions and/or translocations. The pattern of karyotypic stability is consistent with bradytely at the organismic level of evolution. This is explained by a low rate of regulatory genetic evolution promoted by epistatic selection favouring the retention of chromosomal arrangements highly advantageous for overall adaptation. 191 192 0. A. RElG ET AL.