CEEODONTA. 265 



different. It is likely that some of the species of the Bridger formation, to 

 which Marsh has applied generic names, belong to the Creodonta, and may 

 belong to some of the genera described by myself The fact that no generic 

 definitions accompanied the publication of those names, renders their use 

 impracticable. 



ICTOPS Leidy. 



Extinct Mammalia of Dakota aud Nebraska, 1869, p. 352. Proceedings Philadelphia Academy, 1868, p. 



316. 



Dental formula (derived from I. bicuspis where unknown in I. dakoten- 



3 14 3. 



sis) I. -; C. -; Pm. -; M. -. Third superior premolar tooth with two ex- 

 2 1 4 3 



ternal and an internal cusp ; fourth premolar like the true molar, with two 



external tubercles, an internal tubercle, and a posterior cingulum. Fourth 



inferior premolar with an internal and a well developed anterior tubercle ; 



the anterior tubercle of the true molars median in position, and much 



smaller than the internal tubercle. Heels of molars with elevated cusps. 



Orbit not closed posteriorly. Cronoid process of the mandible well 



developed. Inferior margin of mandible not inflected. 



The genus Ictops was determined by Leidy from a species, the /. dako- 



tensis Leidv, from the White River formation. The animals now mentioned 



are identical with it in generic characters, so far as they are ascertained. 



The I. dakotensis is established on a specimen which does not contain all 



the teeth, but the parts preserved indicate that those which are wanting are 



like the corresponding parts of Leptictis Leidy and Mesodectes Cope, with 



which the present species also agree. It is unexpected to identify a genus 



found in the White River horizon with one from the Wasatch Ictops 



agrees very closely with I)idelp)liys. Tlie fourth superior premolar has an 



internal cusp, which is wanting in Didelphi/s, and the infei-ior border of the 



mandible is not inflected. There are also but three superior incisors on 



each side. Under these circumstances I prefer to refer this genus to the 



Bunotheria rather than to the Marsupkdia, but whether its proper place is 



in the Creodont or Insectivorous subdivisions I cannot yet determine. 



