30 Bulletin American Museutn of Natural History. [Vol. XIV, 



V— Family MESONYCHID^ Cope. 

 Triisodon heilprinianus Cope. 



Two very incomplete and poorly preserved skulls of this spe- 

 cies are of especial interest as the first skulls described from the 

 lower or true Puerco Beds, and hence the oldest jnammal skulls 

 knoivn. 



Fig. lo. Triisodon heilfiriniantis Cope. Part of skull, from above, one-half natural size. 

 No. 764, Lower or True Puerco Beds, San Juan Basin, New Mexico. Amer. Mus. Exp. 1892. 



The skull compares most nearly with those of Arctocyon (Cer- 

 naysien), Mesonyx (Bridger and Uinta), and Periptychus (Torre- 

 jon). All have many characters in common : 



Brain small, of low type ; zygomatic arches broad ; occipital and sagittal crests 

 very high ; palatal and basicranial axes parallel (/. e., face not at all bent down 

 on basicranial axis) ; mastoid well exposed, tympanic bulla rudimentary or 

 absent ; basisphenoid broad and slightly convex downward ; glenoid fossa deep 

 and long, post-glenoid process moderately developed ; paroccipital process stout 

 not long, confluent with mastoid, projecting laterally rather than downwards ; 

 muzzle thick and heavy, premaxilla with wide ascending process and long con- 

 tact with nasals. (Nares terminal.) 



All the above characters are primitive ones which will proba- 

 bly be found in nearly all Basal Eocene Placentals. The follow- 

 ing characters, more or less peculiar to Triisodon, I judge to be 

 also primitive. 



