2Sri OARHONIFEROUS FORMATIONS AND FAUNAS <>K COLORADO. 



Among tlio impressions froiii Larimor County hotii a coarsely and a liiicly 

 striated variety can he distin<j;'iiisiied. Tiiese specimens werc^ identified l)y Wliite, " 

 as Sp. f<icl'i/iii(iiitan\iH. Marcoirs name certainly represents a very \'ariable type, 

 to judge l)y ids figures, even if it does not include more than a single species. 

 These specimens, liowever, lia\'e luuisually fine ribs, even f or the Waverly species, and 

 I know no Sjy/'r/yer in the Upi:)er Carboniferous resembling them in other respects 

 which equals them in this. A similar form occurs in the Hermosa formation, but 

 those shells have coarser riljs and arc comparable rather to Sj). hjo/mk, Sp. incrc- 

 hesceim, and the coarse variety of Sp. centronatus. The line-rililjed specimens, how- 

 ever, are not unlike Sj). ■m.bxqtialls, but they differ in having the sinus more sharply 

 outlined, as well as in other particulars. 



In the Crested Butte region this species occurs in relative abundance. The 

 coarselj^ ribbed type predominates, and one specimen is so large and coarsely plicated 

 that I have separated it as a distinct variety. 



In the whitish cherts from near Canyon, Garden Park, and the vicinity, this spe- 

 cies has been collected at nearly every locality and frequently occurs in great abun- 

 dance. The shape varies from subquadrate to subtriangular. The hinge line is as 

 long as the width of the shell and usually somewhat longer. There are 3 to 7 plica- 

 tions in the sinus, according to size, and from 12 to 17 lateral ones. The latter fre- 

 quenth^ manifest a tendency to be broad and flattened instead of sharply rounded, as 

 is characteristic of Sp. centronahis, and occasionally those nearest the fold or sinus 

 are distinctl}' bifurcated. Bifurcation of the lateral plications of typical Sp. centro- 

 natus itself is not unknown, though it is not common. The surface is marked by a 

 system of minute radiating and concentric striae. In most cases the concentric striee 

 alone can be seen, but on well-preserved specimens the radiating ones appear and 

 they then seem to be nearly' as strongly marked as the others. 



The material from the San Juan region is so similar to that from the Madison 

 limestone of Yellowstone Park, and associated with a fauna so manifestlj' allied to 

 that which accompanies the latter, that 1 feel no doubt of their specific identity with 

 one another or with typical Sj>. centronatim from Ohio. 



LooaLity and horizon. — San Juan region (stations 2379, 2381, 2382, 2384); Ouray 

 limestone. Crested Butte district (stations 2350, 2352, 2354, 2355, 2356, 2357); Lead- 

 ville limestone. Salida region (stations 2360, 2361); Leadville limestone. Canyon 

 (stations 2366, 2370); Millsap limestone. Pikes Peak quadrangle (stations 2365, 

 2368. 2369, 2371); Millsap limestone. Castle Rock quadrangle (station 2367); 

 Millsap limestone. Pebbles of Millsap limestone (?) in the Red Beds conglomerate, 

 Larimer Coimty (station 2364). 



aU. S. Geol. Geog, Surv. Terr., Bull,, vol. 5, 1879. p 21.'). 



