566 GEOLOGICAL SURVEY OF IOWA. 



Actinocrinus corniculus (n. s.). 



PLATE X. FIG. 1 a, b, c. 



BODY somewhat urnshaped below, convex above, with a 

 central spine : base small, the inner half of the plates ab- 

 ruptly depressed, forming the cavity for the attachment of 

 the column. First radials and first anal plate extended ho- 

 rizontally, and on the same plane with the lower edges of 

 the basal plates. First and second radials .diverging, and, 

 with the supraradial and brachial plates, lying in one plane 

 from the outer edge of the first radial. Plates all thickened 

 and rising above the suture lines in nodose or tubercular ex- 

 tensions, with short somewhat inconspicuous ridges reaching 

 to the sides of the plates. First anal plate hexagonal, sup- 

 porting three heptagonal plates in the second series ; and 

 above this, several series of smaller plates to the aperture, 

 which is lateral, but near the summit : anal area prominent. 

 First interradial plates nine- or ten-sided, resting on the 

 upper lateral edges of the adjacent first radial plates, bounded 

 laterally by the second and third radials, and on the upper 

 oblique sides by the first arm-plates, and on the summit by 

 the two narrow second interradials or interbrachials which 

 lie between the brachial plates. Arms two from the summit 

 of each ray, composed of a double series of plates. Crown 

 composed of numerous small plates, which are arranged in 

 arches from the summit of each arm. Surface strongly gra- 

 nulose. 



This is one of a small group of species of the Genus ACTINOCRINUS, of which the 

 A. unicornis of OWEN and SHUMARD may be regarded as the type, or rather as ex- 

 pressing the extreme of certain characteristics in one direction, by which the species 

 are allied to the more ordinary forms of ACTINOCRINUS. On the other hand, the 

 gradation through the species fig. 1 and fig. 3 of Plate x show an approach to the 

 AGARICOCRINUS, which is still farther exhibited in Actinocrinus pyramidatus, which 

 is in some respects intermediate between Act. brevis and the Jlgaricocrinus stellatus 

 and Ag. bullatus. In fig. 1 c, Plate x, the basal plates form a narrow rim around the 

 column; while in fig. 3 b of the same plate they are quite inconspicuous, as they are 

 in Actinocrinus pyramidatus, which in this respect corresponds to Jlgancocrinus bul- 



