Boone, Echinodermata, Cruises of "Ara" and "Alva'* 141 



Technical description : The disk is circular, diameter 5 mil- 

 limeters. The abactinal surface has an exceptionally beautiful 

 design, the five pairs of radial plates being very regular and cov- 

 ering the greater part of the surface. Each plate is subtriangu- 

 late, with the outer margins quite convex and the lateral margins 

 convergent to a pointed apex that extends inward three-fourths 

 of the radius of the disk. The surface of the plate is microscopi- 

 cally granulated. The plates of a pair are separated from each 

 other by a row of flat, hexagonal and pentagonal scales. Each 

 scale supports in its center a large, thick, convex tubercle, dis- 

 tally serrulate. The center of the disk is paved with nine to twelve 

 of these irregularly arranged hexagonal and pentagonal scales 

 from which the tubercles have been broken off. Radiating from 

 this center are the five bands of scales arranged in single series, 

 extending one each between the paired radialia. Also branching 

 out from this center, like the spokes of a wheel, are the interradial 

 areas, each of which is as large dorsally as one radial plate and 

 on the circumferal margin appears to be a little wider than the 

 plate, since the interradial area is continuous with the related 

 circumferal margin below the plate. Each interradial area is 

 about two small scales wide at the center and six or seven scales 

 wide at the circumferance. The scales, paving the interradial and 

 circumferal areas without exception, bear each a large convex, 

 distally serrulate tubercle, or bead. 



The actinal surface is paved with scales similar to those of the 

 abactinal center but devoid of tubercles below the circumferal 

 area. The genital slit is relatively large, extending from almost 

 the outer margin of the center mouth plate nearly to the circum- 

 feral margin. The mouth shield is small, widely suboval, with 

 the inner margin produced to a median small point and also with 

 the inner distal margin adjacent to the margin of the tentacle 

 pore of the second under arm plate; the remaining lateral and 

 outer margins of the central plate are widely rounded. The side 

 mouth plates are large, somewhat rectangular with the angles 

 rounded, the inner margins meeting. The outer lateral margin is 

 adjacent to the lateral margin of the small first under arm plate, 

 also the inner angle of the second arm plate. There are ten tenta- 

 cles, two each inside of each small first arm plate. Each jaw 

 angle consists of two small plates separated internally by a nar- 

 row slit, but fused at the apex, where it supports two strong. 



