3()S CARBONIFEROUS FORMATIONS AND FAUNAS OF COLORADO. 



Tbo .sptH'inioiis Ironi the San Juan rogion, though abundant, are indistinct. 

 They appear to Ix-long to the same species as those from Leadville and Canyon. 

 Certain differences in their occun-ence and lithology have led mc to suspect that 

 M. arl-an>tasaiu( occupies a horizon different and somewhat higher than that at 

 which tlie majority of Mississippian species were found in this (San Juan) area. It 

 need not necessarily, however, represent more than a different facies of the same 

 Fauna. 



Although the Leadville material is very imperfect, its characters lend themselves 

 to a similar identiiicatiou. This species appears in the faunal list of the Leadville 

 monograph as Pleurophorus oMongus." With this identification I can in no wise 

 agree. The latter shell has a subquadrate, this a subtriangular, shape. The one is 

 abruptly truncated behind, while in this species the posterior slope is strongly 

 inclined to the cardinal line. The surface, too, is marked bj' rather prominent and 

 regularly disposed lamel'laj of growth, while in PI. oblongus the strise seem to be 

 finer and more crowded. The preservation of the types of PL oblongus, however, 

 is not such as to faithfully portray the surface characters of that species. 



I was at one time disposed to refer my Coloi'ado material to Modiola f nevadensis 

 Walcott, but it can be distinguished from that species by the greater prominence of 

 its umbonal ridge and by the presence of a faint though distinct sinus just in front 

 of the latter in the anterior third of the shell. The shape of the two forms, however, 

 is closely similar. The same characters will serve to distinguish my specimens from 

 Modiola? suhelllptica, which so closely resembles M. ? nevadetuh that the possibilitj' 

 is entertained that they will subsequently prove to be identical. 



M. arkansasana very closely resembles certain shells from the Pottsville series 

 and Lower Coal Measures of the Appalachian region which I have been accustomed 

 to i-efer to Naiadites elmigatiis Dawson. The latter can sometimes be distinguished 

 by having the posterior cardinal angle more nearlj^ quadrate and by having the sur- 

 face marked by finer, sharper strife, but these characters ma}' not hold good in every 

 case. The general resemblance is certainly very marked. 



The fossils from Pella mentioned above can probably be safely identified with 

 M. arkansasana, and afford a satisfactory basis for comparison. From these and 

 from Weller's figures the Colorado form differs in having the axis slightly less 

 oblique and the anterior lobe larger and better defined, producing a gentle emargina- 

 tion in front of the umbonal ridge. These differences are slight in degree, and I am 

 not sure that the two forms should be separated by more than a varietal distinction, 

 if so much. 



The characters which Weller points out as distinguishing his species from 

 Myalina swallowi hold good when the latter is compared with the specimens from 



av. S. Geol. Surv., Mon., yoI. 12, 1886, p. 66. 



