800 THE WHITE RIVER FAUNA. 



stronger than in any other species. The masseteric fossa is deep, and is 

 bounded in front by a prominent ridge which forms the external base of 

 the coronoid process. The direction of this crest is nearly vertical, and its 

 prominence throws the teeth to the inner side of the ramus. There is ^, 

 strong horizontal ridge on the inner side of the base of the coronoid process, 

 in line with the alveolar border, which rises posteriorly and incloses a 

 shallow fossa with the prominent internal vertical edge of the base of the 

 coronoid process. The last molar is situated more than half its length in 

 front of the base of the coronoid process. 



The teeth of this species are about the size of those of P. fugax. The 

 depth of the mandibular ramus is unknown, as its inferior border is broken 

 away. The P. alternans differs from all the species above described in the 

 strong ridges of the coronoid region above described. 



Measurements. 



M. 



Length of two last molars 0038 



Width of third molar 0020 



Elevation of third molar 0018 



From the same locality and horizon as the other species of the genus. 



BUNOTHERIA. 



CREODONTA. 



I refer provisionally to this order and suborder a few species which 

 existed during the White River Epoch in North America. Two species of 

 about the size of the Hedgehog were discovered in the bad lands of Southern 

 Dakota by Dr. Hayden, and were described by Dr. Leidy as Leptidis haydeni 

 and Idops dakotensis in 1868.^ A third species of about the same size, the 

 Mesodectes caniculus, was discovered and described by myself in 1873, with 

 several smaller species already refei'red to the genera Peratherium and 

 Domnina. The three genera first named belong to a distinct division from 

 those of the two genera last named, which is, perhaps, of ordinal value. 

 But this remains uncertain on account of the incompleteness of the speci- 

 mens on which they repose. The entire cranium, without mandible, of 

 Leptidis haydeni is known, and part of the cranium with mandible, and 

 part of the skeleton, of Mesodedes caniculus are in my possession. 



'Extinct Mamm. Dakota and Nebraska, 1869, pp. 345-361. 



