382 CARBONIFEROUS FORMATIONS AND FAUNAS OF COLORADO. 



figured. My identification can not, therefore, be considered conclusive. Tlie Colo- 

 rado sh(>ll iio-roos witli 8wallow'.s description in many particulars, Imt also presents 

 divergencies from it. For instance, the hinge line in tSj>. h(Hmensit< is said to be 

 longer, shorter, or equal to the greatest width of the shell, but in tlie specimens 

 from Colorado this dimension is, I believe, always greater than any other. Swallow 

 also finds no signs of longitudinal strias, but these are alwa^'s minute and faint, and 

 might easily have been obliterated in his material. S;p. hooiiensis, furthermore, 

 is described as being below tiie medium size, which is not true of the Colorado type, 

 and as Sp. hoonensis is compared by its author to Sjy. opinvus without ^\\y discrepancy 

 ill this regard being referred to, I judge that the typical material was considerably 

 smaller than that now in my hands. 



In the comparison alluded to, Sp. Ijoonensis is said to differ from Hp. (ipluniK ''in 

 the form of the area, the greater number and arrangement of the plications on the 

 mesial fold and sinus, and the much greater number on the sides, and I find no signs 

 of longitudinal stria?." It is probable that the absence of longitudinal strife noted 

 is due to individual peculiarities or to imperfect preservation. Hall describes Sp. 

 opimus as having 8 to 10 simple plications, a statement which, as shown by his 

 figures, should be modified so as to mean on either side of the fold or sinus. Swallow, 

 however, could scarcely have understood it thus and written the words quoted above. 

 Discounted in this waj' the differences pointed out by Swallow largely disappear, 

 and one is almost ready to believe with Keyes that Sp. ioonensis belongs in the 

 sj'nonymy of SjJ. opimus li.a\\.— Sp. rockymontanus Marcou. In this case the form 

 under discussion probablj' represents an unrecorded species, for I believe that it 

 should be distinguished from Sp. rockymoiitanus, and am acquainted with but one 

 other Coal- Measure species which invites comparison. I refer to Sp). organensis, to 

 which it seems closely allied. The latter, however, is larger, with more numerous 

 plications both on fold and sinus and on the sides, the lateral ones, moreover, being 

 disposed in fascicles of three or four. The last statement, quoted from Shumard, 

 taken in connection with the rest of the description, would lead to the suspicion that 

 Sp. organensis was a synonym of Sp. cameratus Morton, were it not for the some- 

 what obscure remark to be met with just beyond, that the striae are not arranged in 

 fascicles as in the latter species. 



The aiSnities of the form which 1 have provisionally referred to Sp. Ioonensis, 

 with the Waverly species Sp>. centronatits, are very close. This is so true that it 

 would not be easy on intrinsic characters to distinguish typical examples of the 

 latter species from the Cuyahoga shale of Ohio, from the fossils occurring in Colo- 

 rado. Sp. centronatus is usually a little smaller, a little more alate and mucronate, 

 with two or three more lateral plications, while the minute surface ornamentation, 

 though of the same character, seem to be somewhat coarser and stronger. On the 



