296 PALAEONTOLOGY OF ILLINOIS. 



above the articulating extremity; articulating end moderately 

 enlarged, and provided with a subangular ring, from which it 

 is abruptly contracted; outer extremity terminating in a 

 mucronate point. Entire surface from about three quarters of 

 an inch above the larger end, armed with rather strong, sharp, 

 but short lateral spinules, which are directed obliquely out- 

 wards and upwards. In most cases the surface seems to be 

 entirely smooth, with the exception of the spinules, but by 

 the aid of a magnifier, extremely fine, closely arranged longi- 

 tudinal striae may sometimes be seen on well preserved speci- 

 mens. The articulating end is perforated by a round aperture ; 

 and transverse sections show the central cavity to be compar- 

 atively large for some distance up. 



The only specimens of this species yet obtained consist of 

 detached primary spines, and a few of the interambulacral 

 plates. The latter are wider than long, and apparently nearly 

 smooth, or only ornamented by a single row of small tubercles 

 around the margin; central tubercle prominent, and nearly 

 equaling one-third the greater diameter of the plate, most ele- 

 vated in the middle, which has a small central pit, and is 

 separated from the surrounding ring by a distinct annular 

 groove. Length of one of the interambulacral plates, about 

 0.50 inch; breadth of do., 0.35 inch. Length of a primary 

 spine, 2.60 inches; greatest diameter of the same above the 

 articulating end, 0.16 inch; diameter of head, 0.19 inch. 



The primary spines of this species present much the appearance of the 

 enlarged figure of A. JVbrwoodi, given hy Prof. Hall, on plate 20 of the Iowa 

 Report. They are, however, much more robust, being nearly four, times as 

 large as the natural size of the spines of that species, and also differ in being 

 entirely destitute of granules, between the lateral spinules ; while their longi- 

 tudinal striae are proportionally finer. 



Locality and position : Liberty, Randolph county, Illinois; upper part of 

 Chester group. 



