570 GEOLOGICAL SURVEY OF IOWA. 



middle one largest; above this a prominent line of three 

 successive plates, the lateral ones of each series concealed 

 in a deep groove between the anal ridge and the prominent 

 arm-plates : above these are three series of small plates, 

 the last one surrounding the aperture, which lies against the 

 subcentral apicial spine. A prominent spiniferous tubercle, 

 on each side of the anal opening, projects obliquely upwards. 

 Arms three from each ray adjoining the anal area, and two 

 from each of the other rays, making twelve altogether. 



This species differs conspicuously from A. unicornis in the outline of the basal 

 plates, and the greater prominence and angularity of the other plates, as well as in 

 the character of the spines. The plates of the summit, also, are more pointed; while 

 the plates above each pair of arms, and more especially those above the triple arms, 

 are distinctly spiniform. 



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

 lington, Iowa. 



Actinocrinus snbaculeatns ( n. s.). 



PLATE X. FIG. 2 a, b. 



BODY symmetrically rounded below, abruptly conical 

 above, with an acute spiniform apex : base rounded, and 

 depressed in the centre for the reception of the column. 

 Basal plates gently curving upwards at their extremities, so 

 as to be barely visible when the specimen is viewed lateral- 

 ly. First radials heptagonal, about as long as wide. Second 

 radials quadrangular or pentagonal. Third radials pentagonal, 

 supporting on each side, in the postero-lateral rays, two supra- 

 radials, and in the others one supraradial and a brachial 

 plate. First anal plate large heptagonal, and supporting three 

 comparatively large hexagonal plates in the second range, 

 the third range consisting of three smaller plates ; and above 

 this, three .other ranges of small plates below the anal aper- 

 ture. Summit composed of two series, having about ten in 

 the lower and six in the upper series, surrounding the apicial 

 plate, which is prolonged into a short acute spine. Inter- 

 radial plates hexagonal, the second range sometimes but 



