534 GEOLOGICAL SURVEY OF IOWA. 



This species possesses so many characters in common with P.planus cited above, 

 that I hesitate to regard it as distinct; although there are many points of difference, 

 particularly in the ridges marking the suture of the basal plates, as well as in the 

 deeply excavated upper margins of the radial plate of the original of P. planus. 

 The plates of the two specimens examined are nearly smooth, the fine granula- 

 tions having been worn off ; but in some parts of the surface of the larger plates of 

 fig. 6 a, b, and especially in the plates of the summit, this marking is preserved. 



Fig. 6 a. Lateral view, showing form of cup on one side, radial plates with points of 

 attachment to arm-plates, interradials, and plates of summit. 



Fig. 6 b. View of opposite side, showing similar features in the lower plates with some 

 remains of the arms above. 



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

 lington, Iowa. 



Platycrinus tuberosus (n.s.). 



PLATE VIII. FIG. 7 a, b. 



GENERAL form elevated-hemispherical ; base subpentagonal; 

 cup discoid with the outer edges of the basal plates, the radials 

 and first arm-joints nearly in the same plane ; base deeply 

 concave ; sutures distinct, outer edges bevelled, not as promi- 

 nent as the convex portion of the radial plates. Radial plates 

 subhexagonal, the base and lower lateral margins nearly equal 

 in length, and equal to the width of the upper or brachial ar- 

 ticulating margin. Interradial plates three, the central larger 

 and hexagonal, one of the lateral heptagonal. Central anal 

 plate truncate at the summit, making it heptagonal, or some- 

 times with a small plate adjoining making it octagonal. Sub- 

 brachial plate large, pentagonal, giving origin to a double series 

 of arm-plates which continue to spread in a horizontal direc- 

 tion. Supra-axillary plates numerous. Crown ventricose, com- 

 posed of numerous prominent hexagonal and pentagonal 

 plates, the more central of which are larger, and elevated 

 into prominences. Anal opening about half way between the 

 base and summit, and surrounded by eight or nine small 

 plates. 



SURFACE finely granulose. Column very small. 



