36 MAMMALIA class v 



Prothylacinus Ameghino. |- /. 3P and M^ with well-defined protocone, 

 3P without, M^ with small protocone and metacone, P* slightly larger than P^. 

 Lower ilf with small talonid. Skull long, without alisphenoid bulla. Lower 

 jaw with firm symphysis. Hallux very much reduced. Claws pointed and 

 compressed. Miocene ; Santa Cruz. 



Napodonidis Ameghino. Santa Cruz. Pseudothjlacinus Ameghino. 

 Miocene. Colpodon beds. 



Borhyaena Ameghino (Fig. 57). | /. Stocky teeth, closely set. M'^ button- 

 like. Talonid well developed only on Jf^ and 71/2. Skull short with protruding 

 zygomatic arch. Without alisphenoid bulla. Humerus without entepicondylar 

 foramen. The terminal phalanges blunt, round and with furcate extremity. 

 Miocene ; Santa Cruz. 



Acrocyon, ConodonicHs Ameghino. Miocene ; Santa Cruz. Pseudohorhyaena 

 Ameghino. Miocene. Colpodon beds. Proborhyaena, PharsopJiorus Ameghino. 

 Pyrotherium beds. 



Family 7. Didelphyidae. Opossums. 



Generally small carnivorous marsupials. ■ ' ' ' ' • I small, very closely set. 



C well-developed. Preceding the last, usually very high P, is a long functioning 

 milk-tooth. Upper P tritubercidar and triangxdar, protocone, paracone and metacone 

 y-shaped, usually having a few small basal tubercles along outer edge. Lower M 

 tuber culo-sectorial, with strong metaconid and midtitubercular, well-developed talonid. 

 Extremities five-toed, hallux opposable. Humerus with entepicondylar foramen. 



Both of the existing genera, Didelphys and Chironectes, are found in America 

 ranging from Patagonia to Canada. Fossil representatives are not uncommon 

 in the early Tertiary of Europe and North America, in South America in the 

 Tertiary and Pleistocene. The oldest forms are known from the Laramie of 

 North America. Of the genera Didelphops, Cimolestes, Pediomys, Telacodon and 

 Batodon Marsh, from the L^pper Cretaceous of Wyoming, only isolated teeth 

 and fragmentary jaws are known. 



Didelphys Linn. (Peratherium Aymard, Oxygomphius Meyer, Amphiperatherium 



Filhol, Herpetotherium, Emhassis Cope) (Fig. 58). ' ' ' ' • Besides the three 



principal V-shaped tubercles there exist also on the outer edge of the upper M 



secondary tubercles. Lower 

 M slender. In Europe from 

 the Upper Eocene to the 

 Lower Miocene, in North 

 America in the Eocene, Oli- 

 gocene and Pleistocene. 

 Fig- 5s. Microbiotherium kxne^mo. 



Didelphys {Oxygomphius) frequsns Hx. Meyer. Miocene of /strong. Upper i/ consisting 

 Eckingen, near Ulm. a, 6, c, three teeth from upper jaw, -^/i; d, o f " 010^^^^ 



lower jaw, 1/1 ; e. same enlarged. (After Schlosser.) of three COnical tubercles and 



a simple basal ridge, lower 

 massive with large talonid. Upper Miocene ; Santa Cruz. There also the 

 problematic genera Hadrorhynchus, Stylognathus, Eodidelphys Ameghino. In 

 the Eocene, Notostylops beds of Patagonia, Ideodidelphys Ameghino. Proteodi- 

 delphys Ameghino, supposedly from Cretaceous strata of Patagonia. 



