526 GEOLOGICAL SURVEY OF IOWA. 



The base is composed of three pieces, of which one is 

 quadrangular and smaller than the others, the other two 

 pentagonal ; the whole usually soldered together at their 

 sides, and presenting the form of a flat disk, or a more or 

 less shallow vase-like form. The first radials are large, the 

 lower sides nearly straight, and the lateral margins joining 

 from the base nearly to the upper angles, which are trun- 

 cated for the reception of the interradial and for the anal 

 plates. The second radial, or more properly a subbrachial 

 plate, is inserted upon the excavated upper margin of the 

 first radial, or upon this and a projecting callosity which is 

 more or less distinctly thickened at the margin, and pre- 

 senting a semielliptical or semicircular concave articulating 

 surface for the attachment of the succeeding plates. The 

 second radial ( subbrachial ) plate is often very small, and 

 occupies but a part of the articulating surface of the first 

 radial, allowing the arm-plates to rest in part directly on the 

 first radial on either side, as shown in P. wortheni. This plate 

 may be either triangular or pentagonal, and is succeeded by 

 two series of arm-plates, which may bifurcate once, twice 

 or more, giving origin to a number of arms varying from 

 two to eight or nine from each ray. The arms are ordinarily 

 composed of a double alternating series of plates, which 

 support on their outer edges a series of jointed tentacula. 



In some species the arms extend horizontally, and con- 

 sist of an alternating series of strong articulations, from the 

 upper side of which proceed a series of arm-plates, which 

 are furnished with tentacula as in the arms of ordinary 

 species of this genus. 



Dome composed of numerous small irregular plates which 

 are usually tuberculiform or subspiniferous, and often elevated 

 into a proboscidiform extension : anal aperture lateral or 

 subcentral. 



