394 CARBONIFEROUg FORMATIONS AND FAUNAS OF COLORADO. 



this species, are sometimes so obscured that the shell appears quite smooth. Very 

 small shells in this condition bear a strong resemblance to Amhocodia planivoTwexa, 

 the character which was most used in disting-uishing them being the convexity of 

 the dorsal valve. Some of the specimens from the forks of Cement Creek, in the 

 Crested Butte quadrangle (station 2318), especially, I rather suspect, if more perfectly 

 preserved wtmkl prove to be large examples of Amhoccelia. 



McChesney states that this species seldom attains a greater diameter than five- 

 eighths of an inch. This is true as a rule in the collection studied, but specimens have 

 been noted both from Colorado and the Mississippi Valley which measure 20 mm. 

 (over three-fourths of an inch) and more. Considerable variation has also been noted 

 in frequency of the bands of spines with which the surface was covered. In some 

 specimens the spinous rows are less than 1 mm. apart, and in others they are sepa- 

 rated bj' intervals of 3 mm. Usually the widel}- spaced bands are found upon the 

 larger-sized specimens. All degrees occur between specimens with distant bands 

 and those with proximate ones, and 1 believe that this character can not be given 

 even varietal value. 



Locality and horizon. — San Juan region (stations 2196, 2196a, 2196b, 2196c, 2197, 

 2202, 2208, 2213, 2220, 2221, 2222, 2223, 2224, 2225, 2233, 2238, 2283, 2286, 2301, 

 2332); lower, middle, and upper portions of the Hermosa formation. Ouray (station 

 2194); Hermosa formation. Crested Butte district (stations 2245, 2291, 2292, 2293, 

 2297, 2299, 2300, 2303, 2305, 3306, 2316, 2318); Weber limestone and Maroon forma- 

 tion. Leadville district (stations 2260, 2272?); upjDcr part of Weber formation and 

 Robinson limestone. 



AMBOCCELIA Hall, 1860. 

 Amboccelia planiconvexa Shumard. 



1855. Spirifer planoconvexa. Shumard, Missouri Geol. Sun'., p. 202. 



Upper Coal Measures: On Missouri River, near mouth of Platte River. 



1859. iSpirifer planoconvexa. Meek and Hayden, Acad. Nat. Sci. Philadelphia, Proc, p. 28. 

 Upper Coal Measures: Manhattan, Juniata, and Leavenworth, Kans. 



1860. Amboeelia gemmula. McChesney, Desc. New Pal. Foss., p. 41. 

 Coal Measures: Peoria and Bureau counties. 111. 



1864. Spirifer (Martinia) planoconvexa. Meeli and Hayden, Smithsonian Cont. Knowledge, vol. 14, 

 No. 172, pt. 1, p. 20, figs, a-e, ibidem, 1865, pi. 1, fig. 3. 



Coal Measures: Manhattan and Upper Mill Creek, Kansas. 

 1866. Spiirifer plano-convexa. Geinitz, Carb. und Dyas in Nebraska, p. 42, tab. 3, figs. 10-18. 



Upper Coal Measures, Plattsmouth, Nebraska City, Nebr. 

 1868. MaHinia planoconvexa. McChesne}', Chicago Acad. Sci., Trans., vol. 1, p. 34, pi. 1, 3a-c. 



Coal Measures: Peoria and Bureau counties. 111. 



