698 GEOLOGICAL SURVEY OF IOWA. 



has identified one as A. nerii ? I have not the means of knowing whether the species 

 here described are identical with those he enumerates as new species, or otherwise*. 



Arclueocidaris agassizi ( n. s.). 



PLATE XXVI. FIG. 1 a - d. 



BODY unknown ; plates small, hexagonal, except those 

 adjacent to the ambulacral area, which are a little rounded 

 on that side, becoming pentagonal : central tubercle slender, 

 elongated, tubuliform and projecting above the surrounding 

 annulation, the latter abruptly elevated, and leaving, be- 

 tween its inner face and the central tubercle, a deep cavity. 

 Surface of the plate, immediately around the annulation, 

 elevated in a distinct low annular ridge, beyond which it is 

 depressed and again elevated towards the margin, which is 

 ornamented by a series of low elongated nodes. Spines elon- 

 gated, compressed, contracted below, and swelling out above 

 so that the greatest diameter is about one-third the length 

 above the base ; from this point, very gradually tapering to 

 the summit. Surface of the lower contracted portion, smooth ; 

 muricate above, with small spiniform tubercles, which, on 

 the lower part, are arranged in somewhat distant curving 

 annulating rows, becoming more curved above, or in oblique 

 ascending rows, giving a quincunx order. Point of attachment 

 somewhat elongate, the thickened annulation strongly striate. 



This beautiful species differs from any other American or European species known 

 to me, in its strong compressed spines and comparatively small plates, while in the 

 details of its characteristic markings there is an equal distinction from those here 

 described. 



Fig. 1 a. A fragment preserving several plates of the body, with a small group of 

 the spines, natural size. 



Fig. 1 b. A single plate enlarged. 



Fig. 1 c. Profile view of the same. 



Fig. 1 d. A single spine enlarged to twice the natural size. 



Geological formation and locality. In the Burlington limestone : Burling- 

 ton, Iowa. 



* Dr. SHUMARP has recently described three species : drchtBocidaris aculeatus, jl.bi- 

 angulatus and A. mcgastylis, from the Coal measures of Missouri, Kansas and Texas. 



