Nn. 2.1 COPE ON MIOCENE RODENTIA. 377 



sidered to be conclusive in evidence. It is not very probable that the 

 indications thus obtained will be invalidated by other portions of the 

 skeleton. 



The molars are |, and the crowns support alternating tubercles sepa- 

 rated by shallow open transverse valleys. These are, one on the inner 

 and two on the outer sides of the superior series, and one on the outer 

 and two on the inner side of the inferior. In the recent species, (H. 

 leucoims) there are two inflections on the inner side of the first molar, 

 but in the species here described that tooth is constricted at the posi- 

 tion of the anterior internal loop, and does not regain its width, but 

 continues narrowed to the anterior extremity. The infraorbital foramen 

 is rather large. 



It is probable that there is a second species of this genus in the Loup 

 Fork beds besides the H. loxodon Cope. 



PACIOULUS Cope. 



Paleontological Biilletin No. 31, j). 2, Dec. 24, 1879; Proceedings American Philosopli. 



Society, 1879 (1880), p. 371. 



Superior molars three, rooted. Enamel forming three entrant loops 

 on the external face of the crown, and one on the internal face. 



While the number of the superior molars of Facicnlus is as in the 

 Muridce, the details of their structure is much as in Dasyprocta and 

 Fiber. Two species are known. 



In the P. locJcingtonianus the cranial characters are as follows : The 

 infraorbital foramen is very large, with a general triangular outline. 

 The superciliary borders and temporal ridges are well separated, and 

 there is no sagittal crest. There are no postorbital processes. The 

 otic buUiB are large, and furnished with a very large meatus audi- 

 torius exteruus. The malar is a narrow bone extending to the glenoid 

 cavity posteriorly, and resting anteriorly on a prominent peduncle com- 

 posed of the maxillary bone. It probably reaches the lachrymal. 



This genus is probabl}' one of the Muridce, and a near ally of Sigmodon 

 and Neotoma. It differs from these genera in having three external in- 

 flections of the enamel in the superior molars instead of two. It dif- 

 fers from Hesperomys as these two genera do, viz, in having deep 

 enamel inflections instead of tubercles and valleys. It is true that the 

 deepening and narrowing of the valleys of the molars of Hesperomys 

 would result after wear in a pattern like that of J^eotoma. The same 

 process in Eumys would produce a pattern much the same as that of 

 Paciculus, but that genus is further characterized by the contraction of 

 the postorbital region and the production of a sagittal crest. 



Two species of this genus are known to me, P. insolitus, a smaller, 

 and P. locldngtonianus^ a larger one. Both are from the Truckee beds 

 of Oregon. They demonstrate an early origin for the American type 

 of Neotoma, as contemporaries of the first of the Hesperomys. 



