PALAEONTOLOGY OF IOWA. 



531 



supporting a pentagonal plate, on which the arms subdivide 

 a second time ; each inner division again subdivides, one into 

 three and the other into four, making altogether nine arms 

 from a single ray. Arms composed of a double alternating 

 series of plates which are abruptly wedgeform at their in- 

 terlocking edges, and furnished on their outer margin with 

 slender jointed tentacula. 



SURFACE of plates covered by prominent granulations which 

 are sometimes lamellose, and usually subspiniform. ; often 

 arranged in one or two rows parallel to the edges of the 

 plates, and somewhat irregularly crowded upon other parts 

 of the surface. Joints of the arms in like manner closely 

 covered by fine granulose or ascending subspiniform points. 



This species is remarkable for its flattened or concave base, and the fine closely 

 arranged granulations which cover the entire surface, and are often produced into 

 subspiniform points on the arm-plates, and especially at their adjacent edges : these 

 points are often much extended obliquely upwards into short acute processes. In 

 one specimen these are confluent, and produce thin, extended, and closely arranged 

 lamellae. 



In some respects this species resembles one which I have referred to P. yanddli : 

 but the form of the cup is different, and the granulations always closer and smaller, 

 while the substance of the plates is thin and fragile. The great number of arms is 

 quite sufficient to distinguish the species. It may also be compared with P. granulatus 

 of MILLER (Ds KOXINCK and LE HON, pi. vi, f. 5); but the granulations on the plates 

 of that species are always larger, and to some extent arranged in lines radiating from 

 the angles of the plates; the base is more convex, and the excavation in the summit 

 of the plates for the reception of the arms is much narrower. 



Fig. 4. A specimen preserving the base, with two of the radial plates, and the arms of 

 two series nearly entire. 



FIG. 57. 



The mode of bifurcation of the arms, and the relation 

 of the radial and arm-plates is shown in the accompany- 

 ing diagram. 



