344 CABBONIFERODS FORMATIONS AND FAUNAS OF COLORADO. 



'rii(> apex is one-lialf a diamotcr or a little more I'voiii the iiiaryin and the, height is 

 jiboiil '_' millimeters. The surfaee is marked by sharply elevated, thread-like striae, 

 3 or 4 in a distance of 1 millimeter, with flat interspaces. The latter when exfoliated 

 show faint, tine, radiating lines. Other specimens show that this species attained a 

 diameter considerably' exceeding that stated. 



The measurements and other characters mentioned above, though necessarily 

 somewhat lacking in accuracy by reason of the imperfect nature of my material, 

 indicate that this form is probably that described bj' Meek and Hayden as Discina 

 nianhattanensin. 



Locality and horizon. — Leadville district (stations 22.52, 2264); base of the 

 Weber formation. 



Orbiculoidea sp. 



Near Glenwood Springs (station 2193a) a small species of Orbiculoidea occurs in 

 considerable abundance. Without intermediate forms I do not feel iustified in 

 referring it to the same species as the shells which I have identified as 0. manhat- 

 tanensis, nor have I been able to satisf}^ myself that it can properly be referred to 

 any other known species. It may be briefly described as follows: 



The nearly circular outline has a diameter which rarely exceeds 5 millimeters. 

 The apex is one-third of a diameter, or a little less, from the margin, and the height 

 is sometimes as great as 2 millimeters. In the ventral valve the slit is strongly 

 marked and reaches from near the center to halfway to the circumference. 



The surface is marked hj comparatively strong, somewhat irregular, closely 

 arranged, concentric strise. 



Locality.— (jX3inA River region, Glenwood Springs (station 2193a"^ 



EHIPIDOMELLA Oehlert, 1887. 

 Rhipidomella peoosi Marcou. 



1858. February. Orihis pecosii. Marcou, Geol. North America, p. 48, pi. 6, figs. 14-1 4b. 



Mountain Limestone: Pecos "Village, N. Mex. 

 1858. June. Orthis carbonaria. Swallow, Acad. Sci. St. Louis, Trans., vol. 1, p. 218. 



Middle Coal Measures: Lexington, Mo. 

 1864. Orthis (sp. undet.). Meek, Pal. California, vol. 1, p. 10, pi. 2, figs. 5-5c. 



Carboniferous: Bass's Eanch, Shasta County, Cal. 



1872. Orthis carbonaria. Meek, U. S. Geol. Surv. Nebraska, p. 173, pi. 1, figs. 8a-8c. 

 Upper Coal Measures: Eock Bluff and Nebraska City, Nebr. ; Iowa; Kansas; Illinois. 

 Middle Coal Measures: Lexington, Mo. 



1873. Orthis carbonaria. Meek and Worthen, Geol. Surv. Illinois, Kept., vol. 5, p. 571, pi. 25, fig. 4. 

 Upper Coal Measures: Lasalle, 111. 



1875. Orihis pecosii. White, U. S. Geog. Geol. Surv. W. 100th Mer., Eept., vol. 4, p. 125, pi. 9, figa. 

 5a-5e. ( Volume appeared as a whole in 1877. ) 

 Carboniferous: Santa Fe, N. Mex. 



