NIAGARA LIMESTONE OF CHICAGO, ILLINOIS. 



89 



larger than the second, instead of smaller; the regular interradials are fewer in number, 

 instead of fifteen or over. Lastly, the ridges over the radial plates are entirely wanting in 

 the typical specimens, and in the supposed variety are not interrupted by the sutures. It 

 is quite possible that the structure of the supposed variety may conform completely to that 

 of A. Christyi. 



It will be observed that this species departs from the usual form of Megistocrinus as known 

 in the Carboniferous rocks, at the same time that it falls strictly under the formula of that 

 genus. We feel constrained to say that the same is true of Actinocrinus Christyi Hall, — a 

 circumstance which will enable us to jierpetuate this preoccupied specific name under the 

 form of Ilegistocrinus Christyi. 



Ichthyocrinus Conrad. 



Iehthyoerinus corbis W. and M. 



Body pyriform, section nearly circular, one specimen flattened and 

 even indented on the anal side ; greatest diameter at one third the 

 height of the body below the arm bases; oblique height gradually 

 or rather rapidly curved below, and more rapidly curved in the 

 vicinity of the greatest diameter, giving the body, inmost specimens, 

 a sub-ventricose appearance. Stem very slender at the upper end, 

 turned to one side in all our specimens. Basal plates five, equilater- 

 ally triangular, slightly truncated by the stem at their lower apex, 

 flattened, so that the five sutures by their prominence convert the 

 pelvic cup into an inverted pentagonal pyramid. Radials two in each 

 ray, alternating with the basals, which project their angles in pairs 

 into the lower sides of the first radials, rendering them pentagonal 

 by a reentrant angle below; second radials also pentagonal, with 

 two upper sloping faces to receive the secondary radials. Secondary 

 radials four in each series, each with a transverse diameter equal 

 to twice its height or more; the fourth with two upper sloping sides 

 for the support of the tertiary radials, which are about nine or ten in 

 number in each radiating series, gradually diminishing in height 

 from below upwards, while they increase in transverse diameter, so that the uppermost 

 plates are not more than one twentieth of an inch in vertical height, while their transverse 

 dimension is one fourth of an inch. 



In one of our specimens the height of tlie body to the bases of the free arms is 1.8 inch ; 

 greatest diameter (.55 inch below the free arm bases) 1.62 inch. In another specimen, less 

 ventricose, the height is 2 inches, and the greatest diameter (.66 inch below the free arm 

 bases) is 1.6 inch. 



The distinctive characters of the species are the small size of the stem, the large size of 

 the basal plates, corresponding to the sides instead of the angles of the pentagonal base, 

 the presence of two instead of three radials, and the perfectly straight transverse sutures 

 separating the plates of the several radial series, except the suture separating two succes- 

 sive series. 



In the specimens which we have for examination, the exterior of the plates is removed, 

 and the filling of the sutures projects conspicuously, giving the body a peculiar basket-like 



Iehthyoerinus corbis. 

 Showing the plates of one ray. 



MEMOIRS BOST. SOC. XAT. HIST. Vol. I. 



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