206 Bulletin American Museion of Natural History, [^'ol. XXVII, 



In the U})per Cretaceous of North America occur isolated molars and 

 iaw fragments, apparently representing small species of Didelphoids, which 

 have received the names DideJphops and Pediomijs Marsh (figured in Osborn, 

 1907, p. 96). DideJphops is distinguished by the hypertrophy of the external 

 elements of the crown, especially the parastyle (?) and metastyle (?). Cer- 

 tain specimens of the DideJphops molars are narrow transversely with a deep 

 median indentation of the external cingulum and high crescentic protocone 

 — all very primitive features. The palate is fenestrated (Wortman, 1901, 

 p. 337). The angle of the jaw is sharply inflected. 



The most ])rimitive true Polyprotodont known is the genus Proteodi- 

 deJplujs Ameghino (c/. 1906, p. 288), known from a minute lower jaw from 

 the " ProfeodideJphi/s Beds" (upi)er part of the Chubut formation). This 

 fossil is stated by Ameghino (1906, p. 508) to come from below the Notosty- 

 lops beds, and from lielow the level on which carnivorous Dinosaurs were 

 found. It may therefore be of Upper Cretaceous or of Basal Eocene age. 

 It agrees with DideJpJiis in its dental formula but is distinguished by many 

 very primitive characters and may represent a distinct family. As figured 

 by Ameghino, the condyle is very low, near the level of the cheek teeth; 

 the angle is incom]iletely inflected, apparently not much more than in 

 AmpJiitJierium. The two dental foramina are located as in DideJpJns. 

 The antemolar portion of the jaw is short, the chin heavy. The broad- 

 topped incisors are of the cutting and "cujiped," or posteriorly cingulate, 

 tvpe. The canine is straight and erect, with two fangs; the anterior pre- 

 molars are short antero-posteriorly, but are bifanged; the ultimate premolar 

 is more molariform than in DideJpJiis, having a prominent internal cingulum 

 and a small paraconid and talonid. 'i'he molars have a short talonid with 

 two cusps, a hvpo- and an entoconid. Ameghino (1903, p. 161) notes 

 certain resemblances to the Upper Jurassic Paurodon. While Proteodi- 

 delpJiys is much more advanced than Paurodon, yet in the totality of its 

 characters it seems to carry the Polyprotodont type a step backward toward 

 the generalized Jurassic Trituberculate type. The cupped incisors may be 

 either primitive or secondary, but it is interesting to note that incisors, 

 canines, premolars and molars are somewhat less sharply differentiated in 

 ProteodideJpJn/s than in later forms. 



Thus the palpeontological record indicates a considerable anticjuity for 

 the Didelphid type, but so far does not reveal any of the annectant forms 

 leading to the higher families (except possibly the Caenolestidse, p. 209). 



No Tertiary Dasyuridte are known but the smaller insectivorous forms 

 make a rather close approach to the Didelphid type and the family is dis- 

 tinguished chiefly by the loss of certain primitive characters retained in the 

 Didelphiidte (r/.Bensley, 1903, pp. 90-99, 163-169). 



