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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.

DNA hybridization was used to compare representatives of the major groups of marsupials and a eutherian outgroup. Because of the large genetic distances separating marsupial families, trees were calculated from normalized percentages of hybridization; thermal-melting statistics, however, gave identical topologies for the well-supported clades. The most notable results were the association of the only extant microbiotheriid, Dromiciops australis, an American marsupial, with the Australasian Diprotodontia, and of both together with the Dasyuridae. Estimates of the rate of divergence among marsupial genomes suggest that the Dromiciops-Diprotodontia split occurred -=50 million years ago, well after the establishment of the major clades of marsupials but before deep oceanic barriers prohibited dispersal among Australia, Antarctica, and South America. Because Dromiciops is nested within an Australasian group, it seems likely that dispersal from Australia accounts for its present distribution.Dromiciops australis, the monito del monte of southern Chile and adjacent Argentina, had been considered a small-bodied opossum until Reig (1) noted dental and other similarities to members of the Tertiary family Microbiotheriidae and suggested that Dromiciops is a living representative of this otherwise extinct taxon. Subsequent study of skull anatomy (2) confirmed Reig's proposal, and serology (3) indicated the distinctness of Dromiciops vis-a-vis didelphids. Later, Szalay (4) demonstrated similarities between the tarsi of Australasian marsupials and Dromiciops and proposed that Dromiciops represents the sister group of all Australasian species; together they comprise a monophyletic taxon (Australidelphia) apart from other marsupials (Ameridelphia) in his system. Much recent work has supported Szalay's view: Sharman (5) noted details of Dromiciops' chromosomes comparable to those of several Australasian families; Gallardo and Patterson (6) found male sex-chromosome mosaicism like that in the Australian diprotodontian Petauroides volans; and Temple-Smith (7) documented similarities among the spermatozoa of Dromiciops, phalangers, and kangaroos. Reig et al. (8), however, criticized Szalay's interpretation of tarsal anatomy, and argued that Australasian affinity was incompatible with craniodental evolution, suggesting instead that Dromiciops is the terminus of a lineage that evolved independently of Australasian marsupials.As part of a DNA hybridization study of the relationships of opossums (Marsupialia: Didelphidae), we constructed a matrix including representatives of all major marsupial groups and a eutherian, Procyon lotor. Since one of our aims was to root the didelphid tree, more exemplars of Didelphidae (Didelphis, Metachirus, Monodelphis, Caluromys) were included than of any other marsupial group. Because of the limited range of thermal-melting statistics (9), and the fact that a recent study of Dromiciops using At50H (median melting temperature of hybridized plus potentially hybridizable sequences) (10) did not re...

The primitive topography of the enamelled surface of molar teeth of the Cricetidae is described as expressed in the fossil Cricetodontinae. Morphological variations in the molar structure of different subgroups among the Cricetidae are interpreted as derivations from this cricetodontine pattern. Eleven available naming systems for such components are surveyed, and a new unifying nomenclature is proposed, based on the Cope‐Osborn cusp homologies for mammals. Names for enamel cusps, cuspules, styles, lophs, folds and islands are given, in an attempt to include in an overall general nomenclature the advantages of the most valuable, already available, nomenclatorial systems. The system purports to apply to all modifications of the cricetid crown molar pattern, and it claims to fulfil the need for a uniform nomenclature.

We demonstrate a genetic correlation between rearrangements of the second chromosome of D. buzzatii and thorax length, as a measure of body size. The results indicate that 2j and 2jz3 arrangements are correlated with large size, whereas 2st arrangement is correlated with small size. Some inversions (2st and 2jz3) show dominant effects and others (2j/f3) exhibit overdominance. These results show that at least 25 per cent of body size variation may be accounted for by the studied karyotypes. The possible integration of the genotypic, phenotypic and fitness levels, and also the possible implications to life-history evolution theories, are discussed. These results suggest that, under moderate to high heritability values, some kinds of chromosomal endocyclic and/or balancing selection may be valuable mechanisms for maintenance of body size variation.

The extinct genus Sparassocynus is known from the Montehermosan, Chapadmalalan, and Uquian stages of Argentina, spanning the Pliocene‐Pleistocene transition, and a relative occurs in the older Huayquerian. After a taxonomict review, the morphology of skull, mandible, and dentition is described. The short, triangular face, wide zygomata, broad cranium with large completely bone‐enclosed hypo‐ and epitympanic sinuses, and carnassialized molars are unique among known American marsupials. All these characters are functionally convergent toward living Australian phascogaline dasyurids, small marsupial carnivores, although phylogenetically Sparassocynus clearly represents a moderately specialized offshoot of the American family Didelphidae. It represents a small steppe carnivore now ecologically replaced, at least in part, by placental carnivores that spread from North America in the late Pliocene and Pleistocene.