980 ON A ZOOLOGICAL EXPEDITION TO iiiBAGASOAll. [Dec. 1, 



follows •.—Qricetus is a terminal form amongst its congeners, some- 

 what connected witli the Malagasy NesomyinsB by means of the 

 miocene Cricetodon, from which it is probably directly derived. 



I have lastly to consider the affinities of tlie Malagasy llodentia 

 with the American Ilesperomi/es, urged by Peters with regard to 

 the molars of his genus Nesomys. The resemblance is certainly 

 striking between the pattern of the iViesojnji/s-nioIars and of some 

 of the bunodont Ileqoeromi/iis, 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 and Gricetus and its Old- World allies. On the 

 other hand, the Malagasy llodents preseut unmistakable family 

 features of their own, which all point in one direction, stamping 

 them as lower, more primitive MuridsB 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 the American Hesperomyes 

 and the Malagasy Nesomyes than between the first and the Murinse. 

 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 tlie Myomorpha of South America 

 are recent, having immigrated from the north towards the end of 

 the Diluvium '. The reasons for supposing this are, that so far 

 only hystricomorpbous Eodents have been met with in the older 

 formations, the (Patagoniau and) Santa Cruz beds. Here it must 

 be borne in mind that the presence of a premolar and the pattern 

 of tlie molars approaching tlie llystricomorpha are not sufficieut 

 criteria for assigning to these latter several small llodents of tiio 

 Santa Cruz beds, of which the only parts preserved are the teetli. 

 On the contrary, if tiiere exist forerunners of the Miiridaj in the 

 Santa Cruz beds, they are likely to have possessed premolars. If 

 the ItJdzomyes and the Malagasy Brachyiiromys possessed pre- 

 molars — and there is strong evidence that tliis was recently actually 

 the case — we would be inclined, without knowing more of them 

 than their molar series, to assign them to the Ilystricomorjiha. 



As matters stand at present, it must be admitted that all 

 appearances speak against the ancient domicile of the Hesperomyes 

 in South America, whereas wo have in the miocene of North 

 America such forms as Eumys and others, which might be con- 

 sidered to be the ancestors of the Hesperomyes. Moreover, Crice- 

 todon of the European Miocene is more closely related to the 

 Hesperomyes than to Gricetus. 



There is therefore at present not sufficient evidence of a direct 

 relationship between the Malagasy Eodents and the western 

 Hesperomyes, although it seems to me difficult to explain their 

 alHnities as a result of mere convergence. 



' Uandbuch der Palueootologie, 1, iv. p. 506. 



