378 CARBONIFEKOUS FORMATIONS AND FAUNAS OK COI.oKADO. 



faint, stria' almost obsolete, spines few and scattering, development of sul)niarninul 

 rido'os strong. 



From this it will be seen that 1\ wahunJiemis, especially that variety of it which 

 is chara(^t(n-i>5ed l)y a geniculate curvature, is very similar indeed to P. splen.demi, 

 for P. splf)id,:iis was founded upon a geniculate shell, just as P. wahashensis was 

 founded upon an arcuate one. But in the case of P. splendent the variation to an 

 arcuate t3'pe appears to be less frequent and less pronounced than that in P. ivahmJi- 

 enuLs to a geniculate one. 



On the whole, I believe that P. wabashe9wis, together with its geniculate variety, 

 can be recognized as a distinct species, because of its smaller size, its generallj^ more 

 distinct striation, and its less persistent disposition to develop the Marginifera type 

 of structure. To many, doubtless, it will seem that the geniculate variety of 

 P. wdbashensis should not be separated from P. splendens, and to others, recog- 

 nizing the near relationship of the former with the type to which the name strictly 

 pertains, that the species should lapse to P. splendens as a full synonym. For my 

 own part, as far as my experience extends, it would justify the recognition of these 

 small shells, with their evanescent marginiferoid characters, as at least a variety 

 distinct from P. splendens. The distinction between the arcuate and geniculate 

 phases of P. wahashensis is a minor one and probabh' not worthy of varietal 

 recognition. 



The peculiar beveled condition of the dorsal valve which is present in all these 

 forms, but especially in M. splendens^ was noticed by Norwood and Pratten and 

 shown in their figures." The formation of this truncated rim is correlated with the 

 development of the internal submarginal ridges upon which, chiefly, Waagen founds 

 his genus Margmifera. These occur largely on the dorsal valve and on the sides of 

 the ventral valve. Where specimens possessing this structure are fomid weathered 

 free, the outer or peripheral portions of the shell are usually broken away, so that 

 they ha\'e the appearance of being abruptly terminated at this point. A specimen 

 of this sort is illustrated by Norwood and Pratten (fig. 5a). Their figure, repre- 

 senting a section through the shell, seems to be a constructive one, based upon the 

 abbreviated condition just. mentioned. The dorsal valve, when complete, instead of 

 terminating with the beveled portion and leaving the ventral valve to project 

 beyond, is, as one would expect, prolonged from the inner margin of the bevel, the 

 prolongation conforming with that of the opposite valve and being almost in contact 

 with it. 



In Colorado a certain tendency toward localization seems to characterize the 

 distribution of shells of this group, a feature which, though it maj' be somewhat 

 influenced by doubtful identifications of poor material, without doubt possesses some 



aLoc. cit., pi. 1, figs. 5b and 5c. 



