I30 AMERICAN MESOZOIC MAMMALIA 



Diagnosis. — Sole identified species of the genus. Length M1-4, 7.6 mm. P3, 2.0 

 mm. Depth of ramus internally below M4, 4.0 mm. 



This is far the best preserved lower jaw yet found in the Lance and for that reason 

 it demands taxonomic distinction. It probably belongs with some of the smaller upper 

 teeth now referred, perforce, to Pediomys. The molar type, with its lofty trigonids and 

 very small paraconids, is a strange one for a didelphid and in habitus recalls rather 

 some insectivores. The presence of four molars and the approximation of the entoconid 

 and hypoconulid, among other things, clearly demand its inclusion in the didelphid 

 group, however. The specimen has been fully described elsewhere (Simpson 19293). 



Proteodidelphy s Amtghxno 1898 

 1898. Proteodidelfhys, Ameghino, Rev. Set. (4) X, 74. 



Definition. — Four lower incisors, of nearly equal size. Premolars trenchant, in- 

 creasing rapidly in size from anterior to posterior. Lengths of molars decreasing from 

 Ml to M3. Trigonid longer than broad. Protoconid somewhat higher than metaconid. 

 Paraconid markedly smaller than either. Jaw stout. Incisive region anteroposterior. 



Type. — P. fraecursor Ameghino. 



Distribution. — Supposedly in the late Cretaceous of Patagonia. 



As this is the only identifiable South American mammal which can possibly be 

 regarded as of Mesozoic age, it is fitting that a few lines should be devoted to it here, 

 although the present writer is dependent on the writings of Ameghino for his knowl- 

 edge of the specimen. It is stated to be from the Chubutian or Variegated Sandstone 

 formation of Patagonia, regarded by Ameghino as including the equivalents of the 

 Neocomian and Aptian of Europe. This formation is now believed to belong some- 

 where in the late Cretaceous, and it may be approximately equivalent to the mammal- 

 bearing horizons of the North American upper Cretaceous. It does not appear to be 

 altogether certain, however, that Proteodidelfhys is from these beds, especially in view 

 of the mixture of fossils from different horizons which is now known to have occurred 

 in some of Ameghino's other material. In his detailed description of the specimens 

 (Ameghino 1900) he states that plants were found at one locality, dinosaurs at an- 

 other, and four fragments of mammals at a third locality, not directly associated with 

 any fossils indicative of true Cretaceous age. One fragment was identified as part of a 

 movable plate of an armadillo, cf. Peltefhilus and another as part of a caniniform 

 tooth of a gravigrade. These are not figured and the descriptions do not altogether 

 allay suspicion that they might be reptilian. If correctly identified, they add to the evi- 

 dence that these remains might be from a Tertiary pocket, for the presence of special- 

 ized dasypods and gravigrades in the true Cretaceous would be quite extraordinary. A 

 third specimen is a one-rooted tooth with an enameled crown worn in such a way as to 

 be spatulate. Ameghino considered it as an incisor of a notoungulate and called it 

 Archaeoflus incifiens. So far as one may judge from the figure and description, the 

 tooth is probably but not surely mammalian and is otherwise indeterminate. Proteodi- 

 delfhys itself is a didelphid, more primitive than any living form apparently, but not 



