io8 



THE NEOG^IC REALM. 



[CHAP. 



beds while, as mentioned in the last chapter, they were widely 

 distributed in the northern hemisphere during the Oligocene. If 

 the conclusions of Dr Ameghino are right as to the absence of 

 these marsupials from the Santa Cruz beds, it is evident that 

 opossums only reached South America from the north at the close 

 of the Miocene or commencement of the Pliocene, and that they 

 do not belong to the indigenous fauna. It has been generally 

 considered that the common opossum of the United States is a 

 direct survivor from the Oligocene forms of North America, but it 

 is more probable that it is really a very recent immigrant from the 

 south, seeing that fossil representatives of the genus are unknown 

 from the North American Miocene and Pliocene. During the 

 Miocene the group perhaps survived in the extreme south of 

 North America. 



Although opossums are apparently wanting, the Santa Cruz 

 beds have yielded remains of undoubted marsupials, but several 



FIG. 25. LEFT HALF OF LOWER JAW OF Prothylctcinus, 



nat. size.) 



of them are assigned by Dr Ameghino to a distinct group under 

 the name of Sparassodonta. Foremost of these is the genus 

 ProthylacinuS) already mentioned in the last chapter, which may 

 be provisionally assigned to the Australian Dasyuridce. In having 

 only three in place of four lower pairs of incisors this genus agrees 

 with the latter family, and differs from the opossums ; while the 

 whole character of the lower jaw and dentition is very similar to 

 that of the Tasmanian Thylatinus, with the exception that the 

 premolars are closer together. As in the Dasyuridce generally, 



