380 CARBONIFEROUS FORMATIONS AND FAUNAS OF COLORADO. 



A form from Little Ella Hill (station 2258) in the Leadville region, which is 

 illustrated hy iigs. 12 and 12a of PI. V has been plaeed somewhat doubtfully with 

 2L iiH/fittK. It has a faint sinus and is less strongly arched than in the typical var- 

 iety. Of the lattei', the figured specimen shows an indistinct mesial depression, due 

 in part, at least, to compression and fracture, and the concentric wrinkles and radial 

 strife are unnaturally faint, because they appear upon an internal cast. It is very 

 probable that the shell from Sante Fe, N. Mex., identified by White as P. lorig'i- 

 xjj/'nm Sow., belongs to the species above described; rather less so the similar form 

 from Nevada identified by Meek in the same manner. 



Locality and horizon. — San Juan region (station 2225); middle portion of the 

 Hermosa formation. Ouray (station 2195); Hermosa formation. Leadville district 

 (stations 2251?, 2253, 2259); base of the Weber formation. Crested Butte district 

 (stations 2312, 2315); Weber limestone. Grand River region, Glenwood Springs 

 (station 2193a). 



Marginifera haydenensis n. sp. 

 PI. V, figs. 9 to 9c, 10 to 10b, 11 to 11a. 



Shell small. Ventral valve strongly curved and inclined backward, so that the 

 posterior portion overhangs the hinge line, which is in some examples half waj^ 

 between the posterior outline and the front edge of the shell. The posterior portion 

 is usually flattened, and the curvature geniculate. The width varies from about 

 equal to the length to somewhat greater. The vault of the shell is large. The ears 

 are usually small, but sometimes are large; quadrate, upturned. A more or less dis- 

 tinct mesial sinus is a persistent feature. Surface marked by longitudinal strife, 

 which \'arj^ from strong to almost obsolete, and are very irregular. They are wavy, 

 sometimes bifurcate or fasciculate, often nearly obsolete over the anterior half of the 

 shell. Posterior portion marked by rather regular, faint, concentric wrinkles. 

 Spines comparativelj^ large and moderately numerous. 



Dorsal valve much smaller than the ventral, transverse, with flattened, recurved 

 ears defined bj' ridges. Surface almost without ornamentation, concentric wrinkles 

 being sometimes present, strise rarely. Submarginal ridges developed often to an 

 exaggerated degree. 



Some examples of this shell are almost unique in the extent to which the poste- 

 rior portion overhangs the hinge line, but others are far from displaying this 

 peculiaritjr as well as the specimens figured. This peculiai'ity seems to arise from 

 the condition of .strong fore and aft curvature combined with the late period at which 

 the submarginal ridges are initiated. The character above noted allies Jf. haydenen- 

 sis with M. capacil d'Orbigny, and indeed, its afiinities with that species generally 

 are especially close. The strife of the South American shell are said to be coarse, 

 while in this the}^ are certainly fine. 



