472 CARBONIFEROUS FORMATIONS AND FAUNAS OF COLORADO. 



'•^PoHttlvn and locality, — Froiu a hiird, dark micaceous sandy shale beloiigiiijj- 

 to the Upper Carboniferous series and probat)ly equivalent to the Middle Coal 

 Measures, Ouray, Colo." 



A comparison of the terms of this description with tlie type specimen shows 

 several inaccuracies of expi-ession and one or two additional points to which I desire 

 to call attention. The statement descriljing the mesial liand is misleading, for that 

 structure is not smooth but crossed by distinct sublamellose striae, and it is many 

 times as large as the revolving lines which cross the surface of most shells of this 

 group. In fact, it is proportionately as large if not a little larger than it is usual 

 for the slit band to be. The surface ornamentation consists of transverse and 

 revolving elements which, however, are different in character from one another. 

 The revolving lines are sharph' elevated, threadlike striae which in the specimen in 

 hand are irregular in size and distribution, so that in it the two sides of the shell are 

 differently marked. As shown b}' the figure (fig. 10), the striae which traverse the 

 left side, some 5 or 6 in number, while irregularly distributed, are in every case 

 separated by intervals many times their own width. Between these I think there 

 can be discerned traces of much finer revolving lines. On the other side, the strife 

 are somewhat finer and crowded near the band, but the outer or umbilical portion of 

 this side is marked similarly to the other. Both sides, where the inner end of the 

 whorl is about to be lost in the aperture, are marked alike by fine, crowded striae, 

 among which some, more or less regularl}' distributed, are larger and stronger than 

 the others. On the whole, the surface of the average specimen probably consisted 

 of fine, crowded, revolving lines with larger ones at intervals. Transversely, the 

 shell is marked by many fine-growth lines and by rugte, which are not onl}^ much 

 coarser than the revolving strife, but different in character, inasmuch as they are 

 poorly defined instead of being- sharply and suddenl}^ elevated. They seem to be 

 similar to the rugose elevations which form so marked a character in P. montfort- 

 ianuin, but are more numerous and much less prominent. Growth lines and strias 

 are directed as in the original description, but both can be traced quite to the slit 

 band. The vault of the shell is not of the rounded, circular type, but, being 

 elevated medially and somewhat flattened laterally, the shape of the section is rather 

 that of a Gothic arch. 



This species is related to P. liellvin and P. nodocostatwn, but seems to be distinct 

 from either. I have compared the type specimen of P. ourayeiue with a large series 

 of specimens from Kulo, Nebi'. which probably can be referred to P. helium, 

 and am able to point out the following differences: The peritreme of P. helium is 

 flattened on top with a circular section, while that of P. ourayense is compressed and 

 pointed at the slit band. The surface ornamentation is ver}^ similar in the two 

 species. It differs chiefly in that the transverse rugae are stronger and more sharply 



