PALAEONTOLOGY OF IOWA. 577 



upper edges two other smaller plates ; the third series not 

 distinctly traceable- Anal aperture lateral, near the summit, 

 and surrounded by a series of small plates. Brachial plates 

 two, three or four, resting upon each of the third radials. 



The arms on each side of the anal area in two pairs at 

 their origin, or four from each ray ; those on the anterior 

 ray three, and the intermediate ones in twos, making al- 

 together fifteen arms at the base. Above each set of arms 

 originates a long divergent spine, which gradually tapers to 

 an acute point. A similar spine arises vertically from the 

 centre of the summit, which is surrounded at base by a 

 double series of small plates lying between it and the lateral 

 spines. Surface granulose. 



This species resembles in some respects the Jlctinoerinus (Z)on/mns) mississip- 

 piensis of RCEMEB,; but it is a smaller species, having smooth or granulose plates, 

 with arms varying in number in the different rays, while that one is represented as 

 having four in each. In the species under consideration, the proportional diameter of 

 the base is much greater than in Dr. RCEMER'S species; and they have likewise dif- 

 ferent geological positions. 



There is no reason for separating these forms from ACTINOCIUNUS, so far as I can 

 observe; as we have in different species every degree of development in the plates of 

 the summit, from round to acute tubercles, and thence to spines of varying and ex- 

 treme dimensions, till these appendages themselves become in turn tuberculous or 

 spiniferous*. 



Fig. 12 a. View of anal side of specimen. 

 Fig. 12 b. Anterior side of same. 



Fig. 12 c. Basal view, showing the form and extent of base and distribution of arms. A 

 single spine only is entire, and the others are traced in outline. 



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

 lington, Iowa. 



Actinocrinus pentagonus (n.s.). 



PLATE X. FIG. 6 a, b. 



GENERAL form pentahedral below, with the arm-bases con- 

 spicuously extended : base truncate or slightly concave, 

 composed of three plates ; two equal hexagonal, and one 

 smaller pentagonal plate. First radial plates five, three 



* See Plate xv of this volume. 



[ IOWA SURVEY.] 73 



