BATOCEINID^. 467 



Horizon and Locality. — In the lower part of the Upper Burlington lime- 

 stone, not found above the white crystalline bed, Burlington, Iowa. 

 Type in the Illinois State collection, Springfield. 



Dorycrinus unispinus (Hall). 

 Plate XLV., Fig. m. 



1861. AcHmcritms unispinus— 'B.KVh; Prelim. Desc. New Crinoids, p. 2, and Best. Joum. Kat. Hist. 



p. 270. 

 1873. Bori/mmis unispinus — 'M.sm. and Wokthen ; Geol. Rep. Illinois, Vol. V., p. 380. 

 18S1. Dorycrinus unispinus — W. and Sp. ; Revision Palffiocr., Part II., p. 180. 



Sjn. Actiiiocrinus trinodus Haxl; 1858, Geol. Hep. Iowa, Vol. I., Part II., p. 575. 



Calyx moderately spreading to the top of the second costals, and thence 

 more abruptly to the arms; it is distinctly truncated at the base, and, as 

 seen from above or below, somewhat pentalobate at the arm regions. Ven- 

 tral disk depressed convex, one fourth to one third shorter than the dorsal 

 cup, and crowned by a single large spine. The plates of both hemispheres 

 are not ornamented ; they are virtually flat, with only the apparent convex- 

 ity due to the slight grooving of the suture lines. 



Basals forming a short cup, subcylindrical in outline, often slightly 

 spreading to the lower margin, excavated at the bottom enough to enclose 

 the two proximal stem joints, and pierced by a small pentangular canal. 

 Eadials a little wider than long, as large as the two costals together, the 

 upper face concave. First costals quadrangular, nearly twice as wide as 

 lono-, upper and lower faces convex. Second costals a little wider and 

 longer. Number of arms exceedingly variable, with consequent variation 

 in the distribution of the distichals and the presence or absence of palmars. 

 The anterior and two posterior rays have either three or four pairs of arms ; 

 in the latter case there is but one distichal in both divisions, succeeded by 

 2X2 palmars, but if there are three pairs, the two divisions next the anal 

 interradius have no palmars. The two antero-lateral rays rarely have more 

 than two pairs of arms, and 2 X 2 distichals. Arm openings directed out- 

 ward, arranged in groups; those of the same ray separated from those of 

 adjoining rays by a deep depression. Arms incurving, rather stout for the 

 genus, flattened in their upper parts and serrated at the edges, but without 

 increasing materially in mdth. Regular interbrachials three, rarely four, 

 the upper row on a level with the arm bases. Posterior interradius wider, 

 formed at the upper end into a longitudinal ridge with a deep groove at 



