DESCRIPTIONS OF SPECIES. 329 



This species is very abundant at several localities in the Crested Butte region 

 and it has also been found in the San Juan region. It is massive in its growth, 

 and my specimens indicate that the corallum attained a large size. The cells are 

 small, seldom reaching 0.5 mm. in anj^ diameter. They are usually considerably 

 less. They present an almost infinite variety in the matter of shape, often having 

 reentrant sides, but with the angles almost always rounded. Tabulae are present, 

 but great variation has been remarked in the intervals at which they occur. In 

 some specimens they are regularly distributed and about 0.26 mm. apart. Such 

 examples seem to represent the normal type of Ch. inilleporaceus. In others they 

 are at one place close together, at another quite distant, varying largely in the 

 interval of occurrence, and in still others they can not be detected at all. Part 

 at least of this variation can, I think, be ascribed to obliteration through miner- 

 alizing agencies, for the tabulfe are thinner than the walls, which are regularly 

 preserved, but it is possible that there is considerable real structural variation as 

 well. Periods of alternate growth and inactivity seem to have been an occa- 

 sional feature in the life history of these colonies, which gives the corallum a 

 banded appearance, and near the plane of inanition tabulation seems to be most 

 frequent. The walls are tliick and fibrous, the fibers being- directed at right 

 angles to the cell axes. 



It would seem that our collections show the presence of true Chsetetes mille- 

 foraceus in Colorado, with variations from the normal type, which maj^ be deemed 

 sufficient, provided they are not due to the agencies of fossilization, to constitute 

 a definite varietj^ or possibly even a distinct species. 



Locality and horizon. — San Juan region (stations 2196, 2228); middle portion 



of the Hermosa formation. Crested Butte district (stations 2290, 2306); Maroon 



conglomerate. 



ECHINODERMATA. 



ARCH^OCIDARIS Meek, 1872. 



Akch^ocidakis ouratensis n. sp. 



PI. I, fig. 14. 



The character of this species is well shown by the accompanying figure. It 

 consists of a slender axis, sUghtly over 1 mm. in diameter, upon which are thickly 

 clustered rather short, large spines which have a diameter almost as great as the axis 

 itself. They are subcylindrical, bluntly pointed, inclined upward, and sometimes 

 gently curved. 



The basal and distal portions of the style are unknown. 



This species is so unlike any other American form that comparisons with them 

 are scarcely necessary. Its characteristic features so far as known are, of course, 

 the very large size and close arrangement of the lateral spines. 



Lomlify and horizon. — Ouray (station 2194); Hermosa formation. 



