858 CARHONIFEROUS FORMATIONS AND FAUNAS OK CoLOKADO. 



name ol' tliis iiirgo fiiiiitly lobatc type, especially if, as T hclicvc to ))c tlio case, it 

 can lie correlated with circumstances of range or distribution. 



Locallfi/ and horizon. — San Juan region (stations :i'J2(), 22;-5;i, 2238, 2343); upper 

 part, of the Horniosa formation, Rico formation. Crested Butte district (stations 

 2293, 2303); Maroon formation. Grand River region (station 2324). 



PRODUCTUS Sowerby, 1S14. 

 Productus semireticulatus var. hermosanxjs n. var. 



PI. II, figs. 1 to Ic, 2 to 2b, 3 to 3b, 4 to 4b. 



Although representing a type which has sometimes probably been referred to 

 Productus costatus Sowerby, I believe the material in hand to be more closely related 

 to Prodmctxis sevxireticulatus Martin. Meek" seems to doubt the existence of typical 

 P. costatus in this country, and cites an opinion of Davidson to the effect that some, 

 at least, of the American shells referred to it are more nearlj' allied to Martin's spe- 

 cies. Although the material from Colorado is probably not identical with the form 

 which called forth the remarks just quoted, they tend to corroborate the opinion with 

 which the discussion of this species here opens. 



The stria3 usually number from 8 to 10 in the space of 10 mm. over the median 

 portion of the shell, and are equal and regular, but anteriorly thejr are apt to be une- 

 qual and irregular. In this region tlaey are often coarser owing in large measure, I 

 believe, to the increased size of the shell, their number remaining about the same and 

 their size proportionally augmented. It is not i-are, however, to find two or more stria3 

 coalescing anteriorlj' into a single larger one, nor, on the other hand, is the bifurcation 

 of striae toward the anterior margin of rare occurrence. The former process is some- 

 times regarded as characteristic of Productus costatus, but in the present form it takes 

 place only in a limited degree, and a much greater number of stria? are found around 

 its margin than is the case in P. costatus. Sowerby * cites only 18 in the original 

 description, while in this form 50 or more is a common number. In point of fact 

 however, neither Sowerby's description nor figures indicate that the large size and 

 small number of the striee of P. costatus is due to coalescence, and the reverse seems 

 to be the case. The spines developed in this species, except upon the ears, though of 

 large size, are comparatively few in number. 



On the other hand, though certainly resembling it in many ways, this is not the 

 common form in the Pennsylvanian rocks of the Mississippi Valley which passes 

 under the name of Productus semireticulatus. It is in general somewhat smaller, 

 more inflated and incurved, with fewer and larger spines, and ears which are at the 

 same time more extended and inroUed. These differences seem to me sufficiently 

 constant and well marked to have at least varietal value, and I have distinguished, by 

 the term h^rmosanus, the western form possessing them. This species is related to 



aU. S. Geol. Surv. Nebraska, 1872, p. 159. 6Min. Conch., vol. 6, 1829, p. 115, pi. 560, fig. 1. 



