PALJEONTOLOGY OF IOWA. 673 



Platycrinus sarse* ( n. s.). 



PLATE XVII. FIG. 4. 



BODY broadly turbinate, gibbous in the middle. Basal plates 

 convex exteriorly, and curving upwards from the centre ; 

 upper margins concave ; length about three-fourths as great 

 as the radial plate : column-area large. Radial plates as wide 

 as long, indented on their upper margin with a deep articu- 

 lating scar for the reception of the brachial and arm-plates ; 

 summit of the plates, on each side, reaching as high as the 

 third plate of the brachial series. Subbrachial plate small, 

 triangular, situated in the bottom of the articulating scar, 

 and supporting on each upper sloping side a first and second 

 brachial plate, upon the last of which the plates bifurcate, 

 those of the outer side continuing simple, while the inner 

 adjacent ones bifurcate on the second plate above ; giving 

 six arms from each ray. Arms composed near their base of a 

 single series of wedgeform plates, which gradually become a 

 double series of alternating plates : tentacula composed of 

 joints w r hich are about three or four times as long as wide. 



COLUMN obtusely pentangular, composed of alternating 

 longer and shorter joints. 



SURFACE marked by ridges extending from the base to the 

 upper angles of the basal plates ; and from the centre and 

 outer basal angles of the radial plates, similar ridges con- 

 verge towards the base of the arms ; entire surface granulose. 



This surface character is but obscurely represented in the specimen figured. 



Fig. 4. View of a specimen having the body, arms and tentacula nearly entire, with 

 a small portion of the column. 



Geological position and locality. In the St. Louis limestone, near St. 

 Louis, Missouri. 



* In honorem Dotninse WORTHEN. 



[ IOWA SURVKT.] 86 



