362 OARBONIFKROUS FOKMATION8 AND KAUNAS OF COLOKA I)( >. 



Transverse diameter at the liin^c Hik^ of a inoderato-sizcd iiidixidiml. I'.t nini. 

 Length from hinge line to anterior margin, 15 mm. 



In its shape and proportions this shell belongs to a series of forms among which 

 1 have distinguished live \'arieties or species besides the one in (|uesti<)M. These six 

 species are naturally assembled into two groups, one with line stri;e, the otiier with 

 coarse. Each group in turn separates into three minor units, chiefly by reason of 

 difference in size, and this is ver}' readily accomplished because the differences are 

 strong and not bridged over by intei'mediate examples. Size, however, is not the 

 onlj^ particular, though it is the most obvious one, in which differences can be found. 



Correspondence in size in the two series of forms is striking, so that, assuming 

 this feature as the basis of division, three units would result, each having a coarse 

 and a fine variety. I am not satisfied what importance should attach to the differ- 

 ences marking these six tjqaes, but feel that they are sufficiently strong to deserve 

 notice of some sort. 1 have, therefore, severally referred these forms or phases to 

 species already in the literature which thej^ either resemble or with which they are 

 identical, thus admitting them to a rank which subsequent investigation may con- 

 siderabl}' diminish. 



It surprised me to find this form, which I originally described from strata of 

 Mississippian age in Yellowstone National Park, reappearing in association with an 

 unmistakable Coal Measure fauna, but after a careful comparison of two rather 

 extensive suites of specimens I am unable to point out characters of even varietal 

 value by which they can be distinguished. Only inconsiderable differences, chiefly 

 between individuals, can be discovered, and their identit_y is practically complete so 

 far as shown by my material, which is both abundant and well preserved. 



This form answers verj- closelj' to Swallow's descrijjtion of Pmduetus hoonensis, 

 and 1 suspect, now that its range seems to extend up into the Coal Measures, that 

 my name will prove a synonym. It is impossible, however, to decide this point 

 without authentic specimens of P. hoonensis with which to make comparisons. 



Occasionally, among the material examined, one specimen or two will take on 

 almost the proportions of P. parviforims ^ which differs from P. gaJlatinensis in little 

 but size. 



locality and horizon.— S&n Juan region (stations 2197, 2213, 2216?, 2233, 2247, 

 2284); lower portion of the Hermosa formation. Ouray (station 2195a): Hermosa 

 formation. Crested Butte district (station 2245). 



Peoductus poetlockianus Norwood and Pratten. 



1854. Productus poHlockianus. Norwood and Pratten, Acad. Nat. Sci. Philadelphia, Jour., (2), vol. 3, 

 p. 15, pi. 1, flgs. 9a-c. (Whole volume published in 1855.) 

 Coal Measures: Grayville, White County, 111.; Charboniere, Mo. 



In Sinbads Valley, at station 2285, occurs a form which is identified with some 

 confidence with Norwood and Pratten's Product>/» portJocl'ianus. This species has 



