PALAEONTOLOGY OF IOWA. 575 



lose, with sometimes faint indications of low ridges at the 

 junction of the radial plates. 



COLUMN round, somewhat slender, consisting of alternating 

 longer and shorter joints. 



In the summit, the apicial tubercle and position of the anal opening, this species 

 is analogous with several of the preceding; while the great development of the basal 

 plates gives a very different aspect to the base, and separates this widely from all 

 the other forms. 



Fig. 8 a Anal side of specimen 



Fig 8 b. Anterior side, showing one pair of arms. 



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

 lington, Iowa. 



Actinocrinus trinodus ( n. s.). 



BODY depressed turbinate below, convex above, spreading 

 at the arms : base truncate, the outer margins of the three 

 basal plates projecting beyond the bases of the succeeding 

 radials. First radial plates five, two heptagonal and three 

 hexagonal, nearly twice as wide as long. Second radials 

 small, quadrangular, twice as wide as long. Third radials 

 pentagonal, twice as wide as long, and, on the anterior and 

 antero-lateral rays, supporting the brachial plates on their 

 upper oblique sides ; while in the postero-lateral rays there 

 is an intermediate supraradial plate on one of the divisions, 

 and two on the other (there being an irregularity on one 

 side). First interradial plate octagonal, supporting two some- 

 what irregular hexagonal plates, and a second series of two 

 which lie above the base of the arms. First anal plate hep- 

 tagonal, supporting in the second range a central hexagonal 

 plate, which, with the two succeeding similar ones, forms a 

 prominent round elevation reaching to the small plates sur- 

 rounding the anal opening. In the second anal range there 

 is one lateral heptagonal or hexagonal plate on each side ; 

 in the third range, two on each side ; and in the fourth 

 range, two on each side of the central plate : base of anal 

 opening on a plane with the upper margin of the arm-bases. 



