MYOMORPHA. 853 



No. 2), .0064; length of first molar, .002; width of mcisor, .001; depth of 

 ramus at second molar, .004. 



The Hesperomys nematodon is about as large as the red squirrel, Sciurus 

 hudsonius, and considerably larger tlian the II. loxodon. 



PACICULUS Cope. 



Paleontological Bulletin No. 31, p. 2, Dec. 24, 1879. Procoedings American Philosoph. Society, 1379 



(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 Paciculus is as in the 

 Murulce, the details of their structure is much as in Basyprocta and Fiber. 

 Two species are known. 



In the P. lockingtonianus 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 bullae are 

 large, and furnished with a ver}^ large meatus auditorius externus. The 

 malar is a narrow bone extending to the glenoid cavity posteriorly, and rest- 

 ing anterioi-ly on a prominent peduncle composed of a maxillary bone. It 

 probably reaches the lachrymal. 



This genus is probably one of the Muridce, and a near ally of Sigmodon 

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

 tions of the enamel in the superior molars, instead of two. It differs 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 narrow- 

 ing of the valleys of the molars of Ilesperomys would result after Avear in 

 a pattern like that of Neotoma. 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 produc- 

 tion of a sagittal crest. 



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

 P. locliingtonianus, a larger one. Both are from the John Day beds of Ore- 

 gon. They demonstrate an early origin for the American type of Neotoma, 

 as contemporaries of the first of the Ilesperomys. 



