SINCLAIR: MARSUPIALIA OF THE SANTA CRUZ BEDS. 441 



No. 15,425. No. 1 



Posterior premolar, transverse diameter .0008 



My, antero-posterior diameter ...... .005 



" transverse " ...... .0023 



height of middle of crown, above alveolar border . . .0045 



Mj, antero-posterior diameter ...... .0033 -0033 



" transverse ' OO22 .0025 



Mj, antero-posterior ' 003 .003 



" transverse ' 002 .002 



M-f, antero-posterior ' ...... .002 



" transverse ' ...... .0017 



Depth of mandible below middle of sectorial .... .0065 .0065 



" " " M ...... .006 .006 



RELATIONSHIPS OF THE OENOLESTIDyE. 



The most primitive Santa Cruz representative of the Caenolestidae is, 

 undoubtedly, the genus HalmarhipJnts, which represents, with little or no 

 modification, a type which is not only ancestral to the Palaeothentinse, but 

 agrees perfectly with the "minute insectivorous forms which, apart from 

 the diprotodont modification of the antemolar teeth, possessed a full ante- 

 molar formula," indicated by Bensley's studies as the ancestors of the 

 Phalangerinae. In this interesting genus the dental formula and molar 

 patterns are didelphid, affording striking evidences in favor of the theory 

 of the didelphid origin of the Diprotodontia (see Bensley, 1903). It has 

 already been shown (p. 417) that the lower molar patterns in the Palaeo- 

 thentinae are readily derivable from the Halmarhiplms type by a reduc- 

 tion in height of the cusps and the formation of cross crests. So far as 

 our knowledge of Halmarhiphus warrants an inference, there can be little 

 objection to deriving the Palaeothentinse from a similar ancestral form. 

 The chief objection to regarding Halmarhiphus as directly ancestral to 

 the Palaeothentinae arises from the fact that the latter are represented in 

 formations older than the Santa Cruz (P. chubtitensis from the Pyro- 

 therium beds, Amegh., 1897, P- 9^)- That Halmarhiphus is in the direct 

 line of descent culminating in Ccenolestes will, it is believed, hardly be 

 questioned after an examination of the accompanying plates. So far as 

 the lower dentition is concerned, no argument can be advanced to the 

 contrary. The recent genus shows a slight tendency toward the develop- 

 ment of crescents, while, in its Santa Cruz predecessor, the cusps are high 



