568 GEOLOGICAL SURVEY OF IOWA. 



Actinocrimis unicornis. 



PLATE X. FIG. 5 a, b, c. 



Actinocrinus unicornis : OWEN and SHUMARD, Geological Report of Iowa, Wisconsin and 

 Minnesota, 1852, pa. 593, Tab. V A, f. 12. 



BODY turgid, somewhat elongate-spheroidal : base trun- 

 cate, and summit crowned by a strong pointed spine ; base 

 distinctly tripartite, depressed in the centre and margined 

 by a thickened rim, which is notched at the'junction of the 

 three plates. First radial plates heptagonal, massive, tumid, 

 and angularly tuberculiform in the centre. Second radial 

 plates quadrangular, twice as wide as long. Third radials 

 small pentagonal, each supporting on its upper oblique sides 

 two simple brachial plates upon which rest the arms. First 

 interradial plates nine- or ten-sided, tuberculiform, resting 

 on two adjacent first radials, and supported laterally on each 

 side by the second and third radials ; while the first brachial 

 and sometimes the second brachial plates rest upon the 

 upper sloping sides, and the truncated summit supports a 

 narrow second interradial, which is, perhaps, sometimes 

 divided. The anal group consists of one, three, and three 

 plates ; the two lateral plates of the third series supporting 

 each a superaxillary plate, and the central one lying between. 

 Anal area pointed upwards, very turgid, covered by nume- 

 rous small plates ; the last series consisting of six, and resting 

 directly against the base of the apicial spine. Plates of the 

 summit tuberculiform ; the series below the apex consisting 

 of six, besides those connected with the aperture. Arms three 

 from each ray adjoining the posterior area, and two from each 

 of the others. Surface granulose. 



This species is not rare : it presents some variations in character, particularly in 

 the prominence of the plates, which are usually protruded in the centre in the form 

 of nodes or tubercles. It differs from all the other spiniferous species which I have 

 observed, in the character of surface, and in the close proximity of the anal opening 

 to the central spine, its upward direction, and great proportional elevation above the 

 bases of the arms. 



