Acta Geologica 3. (1955)

4. szám - Kretzoi, M.: Dolomys and Ondatra

DOLOMYS AND ONDATRA 353 ing that these genera are not closely related to the Prometheomyinae which, known in a single genus only, show a great similarity to the Ellobiinae as far as dentition is considered, hut are very different in other respects (cranial characters etc.). This problem cannot be solved until we get further information about the skull of the above mentioned Hungarian and German «preglacial» forms. Till then, Prometheomys and Ellobius should strictly be held apart. Two further very distinct types, represented by Lemmiriae and Myospala­­cinae, are much more suggestive of the ancestral molar structure of the Barano­­myinae. The cranial structure and general build of Myospalacinae, have deviated from the usual shape of voles to such an extent that they are generally not referred to this group, although — on the other hand — the basic pattern of their dentition points to ancient connections with that of Microtodon. The Lemmings (exclusive of Dicrostonys which is usually but wrongly classed in this group) form another very exclusive group the dentition of which shows the modified characters of the ancestral structure even more clearly than that of the Myospalacinae. If we put all the above treated lines — the Hesperomyinae, ancient Barano­­myinae, the Lemminae which have retained the primitive but, in other regards, differently developed pattern of the grinding surface, the distinctively special­ized Myospalacinae, further the Ellobiinae and Prometheomyinae which have branched off in an independent manner, and finally the Microtoscoptes which probably form but a short blind branch — on one side, we may put all the other voles, i. e. the great majority of Arvicolidae on the other side, since they belong more or less uniformly to a further branch characterized by a complication of the frontal prism (so-called «cap») of Mt. In consequence of the great variety, the Arvicolinae can be divided into separate groups, all of which show differing characteristics regarding the trend and evolutionary degree of their representatives. The first group comprising Promimomys, Mimomys as well as the genera Arvicola and Phaiomys-Allophaio­­mys ( Arvicolini) show the most ancient type of molar structure, retained as a conservative pattern even in the living forms. The second, very closed group of Arvicolinae includes the tribe Ondatrini which has been dealt with above. The third group, Microtini, including the bulk of the genera of the family characterized by a mediocre degree of development, may be held, in spite of its great variability, as a closed unit. The fourth and fifth groups are formed by two phylogenetically isolated genera, namely Lagurus and Dicrostonyx respectively (Lagurini and Dicrostony­­chini) ; the dentition of the former showing a moderate development, that of the latter reaching a very high degree. Of the groups which did not appear until the beginning of the Quaternary, that of Lemminae is represented since the onset of the North American and

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