536 GEOLOGICAL SURVEY OF IOWA. 



Platycrinus sufospinosus (n. s.). 



PLATE VIII. FIG. 9 and 10. 



GENERAL form of body depressed, semiorbicular. Calyx dis- 

 coid, decahedral, flat below. Basal plates anchylosed ; the 

 sutures marked by simple lines, broadly and deeply depressed 

 towards the centre, edges bevelled. Radial plates subhexa- 

 gonal, extending for half their length almost in the same 

 plane with the base, or at right angles to the axis, where 

 they are suddenly bent upwards at the sides : margins (es- 

 pecially of the base) bevelled, producing a groove along the 

 sutures. Subbrachial plate resting upon the truncated extre- 

 mity of the radial plates, and continued in the same horizon- 

 tal plane. Interradial plates five or six-sided : anal plate 

 eight or nine-sided ; anal orifice ( which is imperfect ) bor- 

 dered by numerous minute plates. Axillary plates small, six- 

 sided. Supra-axillary plates in two ranges : plates of the 

 summit produced in the centre into short, obtuse spines. 



SURFACE very finely granulose. Column comparatively 

 large, round at the summit. 



This species is closely related to P. tuberosus, but is smaller, less ventricose above, 

 with the plates of the summit produced into short thick spines instead of rounded 

 tubercles; and there are also minor differences in the form of the interradial plates, 

 and in the proportional size of the column. It resembles P. discoideus in the discoid 

 calyx; but, besides being more extended in the plane of the base, the surface is not 

 rugose. 



Fig. 9. View of a small specimen, showing the base and first arm-joints. 

 Fig. 10. Profile view, showing the surface of attachment for the second arm-plates, the 

 anal aperture, and the spiniform extension of the plates. 



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

 lington, Iowa. 



Platycrinus sculptus (n.s). 



PLATE VIII. FIG. 11. 



BASE basin-shaped ; sides spreading nearly straight, very 

 distinctly pentagonal ; margins very slightly concave, some- 

 what abruptly depressed at the centre with edges of the 

 depression salient. Column at point of attachment small. 



