80 SINCLAIR— FAUNA OF SANTA CRUZ BEDS. [April 13, 



the last upper molar has undergone greater reduction and the sty- 

 loid cusps have decreased in number, the antero-external alone being 

 represented. Apart from these advanced characters in the denti- 

 tion, the Santa Cruz thylacynes are of a distinctly more primitive 

 type than their surviving Tasmanian relative, which has progressed 

 in the lengthening of the face and posterior shifting of the orbit, 

 the increased brain capacity, the acquisition of palatal vacuities, 

 the prenatal shedding of the deciduous teeth, the external shifting 

 of the outer cuneiform, and the loss of the hallux. With the ex- 

 ception of the reduced hallux in Prothylacynus, transitions to these 

 advanced types of structure do not appear in the Santa Cruz mem- 

 bers of the family. 



The marsupial faunas of those formations in Patagonia older 

 than the Santa Cruz are still too imperfectly known to afford a 

 secure basis for phylogenetic speculation, but it may confidently 

 be expected that the common ancestor of Thy lacy mis and the ex- 

 tinct Santa Cruz types will be found among them. In fact, certain 

 large carnivorous marsupials from the Pyrotherium beds named by 

 Ameghino, Proborhyama and Pharsophorus retain the metaconid in 

 the lower molars as in the Dasyuridai, while the premolar formula 

 is unreduced as in the Thylacynidre. 



The affinities of Microbiotheriimi are unquestionably didelphyd. 

 The genus can not be regarded as ancestral to any of the existing 

 South American opossums as the degree of reduction of the ex- 

 ternal cingulum and styloid cusps in the upper molars is greater. 



The most primitive of the Csenolestidae, the genus Halmarliiphus, 

 is transitional to the Polyprotodontia and represents, with little or 

 no modification, a type which is not only ancestral to the Palreo- 

 thentinae but agrees perfectly with the "minute insectivorous 

 forms which, apart from the diprotodont modification of the ante- 

 molar teeth, possessed a full antemolar formula," indicated by 

 Bensley's 1 studies as the ancestors of the Phalangerinae. Un- 

 fortunately this interesting transitional genus is known only from 

 the lower jaw. The Palaeothentinae are important in retaining 

 constructive stages in the evolution of the bunodont type of molar 

 characteristic of the more primitive of the existing phalangers. 

 The Abderitinas are highly specialized diprotodonts which appear 



1 Bensley, B. A., "The evolution of the Australian marsupials, etc.," Trans. 

 Linn. Sec, London, ser. 2 (Zool.), vol. 9, p. 1 39, 1 903. 



