PALAEONTOLOGY OF IOWA. 527 



Platycrinus cavus (n. s.). 



PLATE VIII. Fia. 1 a, b. 



Compare Platycrinus corrugatus, OWEN and SHUMARD, Geology of Iowa, Wisconsin and 



Minnesota, pa. 589, pi. V A, f. 2 a - d. 



GENERAL form of the calyx a shallow vase, the basal plates 

 scarcely extending above the plane of the margin of the 

 second range when viewed in profile. Basal plates forming 

 a pentagon, with the sutures closely united ; the line of 

 junction often indicated by a shallow groove, gradually de- 

 pressed from near the margin to the centre, which is perfo- 

 rate. First radial plates moderately expanding above, length 

 and width about as three to four ; slightly excavated above, 

 with a semicircular articulating scar for the reception of the 

 second radial plates ; length of the articulating portion less 

 than half the length of the plate, limited below and at the 

 sides by a strongly elevated ridge ; lines of the sutures 

 marked by a distinct angular groove. Second radial s and 

 arms unknown. 



SURFACE marked by concentric lines of nodes or granules, 

 which are sometimes confluent. About two or three ranges 

 of nodes parallel to the margins mark the basal plates out- 

 side of the circular depression, with a more distinct row 

 running to each angle of the plate. Surface of radial plates 

 marked in like manner by two or three ranges of nodes pa- 

 rallel to the basal and lateral margins, with a distinct row 

 of confluent nodes from the lower angle towards the articu- 

 lating scar. A plain space, equal to the granulate portion, 

 lies between the latter and the elevated ridge bounding the 

 second radial. 



This species bears much resemblance to the P. coirugatus of OWEN and SHUMARD; 

 but the form and proportions of the calyx are different, and also the character of the 

 articulating cicatrix, as well as the form of the first radial plate The surface markings 

 in this species are distinct nodes or granules in regular rows or lines, reaching but 

 half way from the base of the first radial to the base of the second, leaving a smooth 

 finely granulose surface of equal width ; while in the P. mrrugatus , the rugose character 

 extends to the base of the second radial. There are in the collection some separate 



