FORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 253 



7GIDLEYINA SILBERLINGI (Gidley) 15 



Figure 73 



IGidleyina silherlingi (Gidley), Simpson, 1935d, p. 240. 



2VP6.— U.S.N.M. no. 6166, partial left lower jaw with P3-M3. 

 Collected by A. C. Silberling. (In the same lot are a partial right 

 lower jaw with M2-3 and another right lower jaw fragment with M2 

 and the heel of Mi. They probably belong to the same species but 

 include parts of one or two different individuals and are excluded from 

 the type material.) 



Horizon and locality. — Loc. 27, about 400 feet above the base of 

 Fort Union No. 3, Wheatland County, Mont.'*^ 



Diagnosis. — Gidley: " ... About the size of or a little smaller 

 than E. minor [= Tetraclaenodon pliciferus, G. G. S.] . . . Jaw rela- 

 tively long and slender, especially anteriorly; the teeth proportionately 

 narrow transversely . . . with a decided tendency to selenodonty 

 . . . The paraconid in the molars is vestigial or wanting, and P4 is 

 submolariform . . . the heel . . . having the crescentic form of that 

 of the molars, while the metaconid is large and as high as the 

 protoconid." 



Discussion. — It is possible that this is the lower dentition of Gid- 

 leyina montanensis. Since, however, it cannot be demonstrated to 

 belong even to this genus and since among lower dentitions it is a 

 distinctive and interesting type that requires some means of reference 

 until its association with upper teeth can be established, it seems quite 

 proper to accept Dr. Gidley's decision to define it as a species, which 

 can be reduced to synonymy later, if necessary, with no great con- 

 fusion. 



In comparison with other known low^er jaws, this is generically dis- 

 tinct from any previously described. Ectocion is similar but has a 

 simple longitudinal crest on P3, instead of an incipient crescent, while 

 P4 is more complicated and molariform, with a distinct posterointernal 

 cusp absent in the present specimen. The molars offer no contrast 

 definitely of generic value, unless it be the somewhat larger and more 

 definitely closed trigonid basins and less distinct vestigial paraconids 

 of IGidleyina silherlingi. The possibility that Gidleyina is not really 

 ancestral to Ectocion or, on the other hand, that ?6r. silberlingi does 

 not belong to Gidleyina is enhanced by the fact that whereas the upper 



" In a draft of the manuscript on this family, Dr. Gidley describes this as a species of Euprotogonia 

 (= Tetraclaenodon) . On the specimen label he has crossed out "Euprotogonia" and written "Ectocion." It is 

 thus evident that he recognized the probable relationship of this jaw to the new genus I have named Gid- 

 leyina, since this was also successively identifled by Dr. Gidley as Euprotogonia and as Ectocion before its 

 generic distinction was recognized. I have not quoted his diagnosis in full, giving only enough to validate 

 his claim to authorship of the species, since it was written before he had recognized the genus here named 

 Oidleyina and therefore is not fully apropos. My comparisons following the diagnosis suffice for the expres- 

 sion of more fully studied opinion as to diagnosis and affinities. 



" Given on labels, etc., as "Sweetgrass County", but, as can be seen on the map, this is one of several 

 localities slightly north of the county line. 



