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Show 980 ON A ZOOLOGICAL EXPEDITION TO MADAGASCAR. [Dec. 1, follows :-Cricetus is a terminal form amongst its congeners, somewhat connected with the Malagasy Nesomyinae by means of the miocene Cricetodon, from which it is probably directly derived. I have lastly to consider the affinities of the Malagasy Rodentia with the American Hesperomyes, urged by Peters with regard to the molars of his genus Nesomys. The resemblance is certainly striking between the pattern of the Nesomy s-molavs and of some of the bunodont Hesperomyes, and this resemblance extends alike to the form of the skull in both groups (conformation of the boundaries of the infraorbital foramen, small size of the tympanies, &c). There exists more agreement between these two groups than between them aud Cricetus and its Old-World allies. On the other hand, the Malagasy Rodents present unmistakable family features of their own, which all point in one direction, stamping them as lower, more primitive Muridae than the Hesperomyes. The two anterior, and in some cases all three, molars are more like each other in size and pattern, although in this respect the difference is much less striking between tbe American Hesperomyes and the Malagasy Nesomyes than between the first and the Murinae. Similar remarks apply to the skull: the infraorbital foramen is larger in the Malagasy mice, the malar bone always stronger developed and extending farther forwards and upwards towards the lachrymal &c. Zittel is of opinion that all the Myomorpha of South America are recent, having immigrated from the north towards the end of tbe Diluvium l. Tbe reasons for supposing this are, that so far only hystricomorphous Rodents have been met with in the older formations, the (Patagonian and) Santa Cruz beds. Here it must be borne in mind that the presence of a premolar and tbe pattern of the molars approaching the Hystricomorpha are not sufficient criteria for assigning to these latter several small Rodents of the Santa Cruz beds, of which the only parts preserved are the teeth. On the contrary, if there exist forerunners of the Muridae in the Santa Cruz beds, they are likely to have possessed premolars. If the Bhizomyes and the Malagasy Brachyuromys possessed premolars- and there is strong evidence that this was recently actually the case-we would be inclined, without knowing more of them than their molar series, to assign them to the Hystricomorpha. As matters stand at present, it must be admitted that all appearances speak agaiust the ancient domicile of the Hesperomyes in South America, whereas we have in the miocene of North America such forms as Eumys and others, which might be considered to be the ancestors of the Hesperomyes. Moreover, Cricetodon of the European Miocene is more closely related to the Hesperomyes than to Cricetus. There is therefore at present not sufficient evidence of a direct relationship between the Malagasy Rodents and the western Hesperomyes, although it seems to me difficult to explain their affinities as a result of mere convergence. 1 Handbuch der Palaeontologie, 1, iv. p. 556. |