l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I23 



therefore, offers no clue as to the specific age of the sediments from 

 which it was recovered. 



Class MAMMALIA 



Order Rodentia 



Family ISCHYROMYIDAE 



Subfamily Sciuravinae 



FLORESOMYS Fries, Hibbard, and Dunkle, new genus 



Genotype. — Floresomys guanajiiatoensis. 



Diagnosis. — Dentition %, %, %, % ; crowns of cheek teeth slightly 

 higher than those of Sciuravus and Taxymys. P^ is reduced and 

 pointed. There is no evidence of a cingulum. P"* is molariform and 

 smaller than M^. The paracone and protocone, also the metacone 

 and hypocone of P*-M- are distinct and well developed. The para- 

 cone and protocone of these teeth are distinctly separated from the 

 metacone and hypocone by a deep transverse median valley. The 

 mesostyle is lacking on P*-AP. The teeth have an indistinct basin 

 betvi^een the anterocrest and the posterocrest. A true metaconule is 

 absent. A small conule (more properly called a hypoconule) is 

 present at the anteromedian base of the hypocone of P"*— M". The 

 lower cheek teeth are lacking a mesoconid and mesostylid. The proto- 

 conid, metaconid, hypoconid, and entoconid are distinct and well devel- 

 oped, the basin between the cusps is reduced and the transverse valley 

 is deep, separating the tooth into distinct anterior and posterior parts. 

 The masseteric fossa extends forward as far as the middle of Mi. 



FLORESOMYS GUANAJUATOENSIS Fries, Hibbard, and Dunkle, new species 



Type. — I.G.M. No. 52-137, Instituto de Geologia de la Univer- 

 sidad Nacional Autonoma de Mexico, associated rami, each bearing 

 part of the incisor and P4-M3 ; and the left maxillary bearing P^-M^, 

 collected in March 1952, by Carl Fries. Paratype: U.S.N.M. No. 

 20139, fragmentary left ramus bearing P4-M3 of a young adult 

 rodent, collected in October 1952 by David H. Dunkle. 



Horizon and locality. — Thin sandy silt lens in red conglomerate 

 about 2,000 meters south of the town of Marfil, Guanajuato, Mexico. 



Description (fig. 6). — The lower jaws and associated left maxillary 

 are those of a young rodent. P^ is so greatly reduced that it is not 

 functional at this stage of wear. The surface of its crown is below 

 the base of the anterior cingulum of P^ P*, though reduced in size, 

 is molariform. The anterior cingulum is short, connecting with the 



