430 BULLETIIT 100, UNITED STATES NATIONAL MUSEUM. 



ECHINASTER STEREOSOMUS Fisher. 



Plate 122, figs. 2, 3 ; plate 132, figs. 6, 6a-e. 

 EcMnaster stereosomus Fishee, 1913&, p. 195. 



Diagnosis. — Eays 5. K=52 mm., r=10 mm., R=5.2 r; breadth of 

 ray at base, 12 mm. Disk fairly large; rays tapering at first rather 

 rapidly, then very gradually, the distal two-thirds being attenuate; 

 abactinal surface of rays well arched, actinal surface slightly convex. 

 Spines short, conical, sharp, spaced 2 to 4 times their length, forming 

 2 marginal and proximally 5 (distally 3) abactinal, longitudinal 

 series; papulae, 1 to 5 to each of the roundish, or more or less quad- 

 rate areas; no intermarginal or actinal papulae; skin fairly thick, 

 covering everything except the tips of spines, and crowded with 

 minute perforated plates and rods; a few true actinal intermediate 

 plates present, all but 1 (which bears a spine) rudimentary; adam- 

 bulacral armature a vertical series of 3 spines, the uppermost on 

 border of furrow, the lower deep in furrow ; upper spines with their 

 membrane investment forming a continuous serrate web closing 

 mouth of furrow. Eelated to E. callosus, but with larger disk, shorter 

 rays broader at base, shorter spines, 3 adambulacral spines, and 

 thinner skin, crowded with much more numerous, evenly distributed 

 perforated plates of a different form. 



Description. — Abactinal skeleton rather open, forming quadrate or 

 roundish meshes, becoming elongate near end of ray. Most of the 

 nodal points bear a short, stout, conical spine on an eminence of the 

 plate. These spines form at base of ray 5 abactinal, 1 lateral, and 1 

 actinal series, the last two borne by the superomarginal and infero- 

 marginal plates, respectively. The abactinal series adjacent to 

 superomarginals ends at about the distal third of ray, so that beyond 

 this point there are only 3 abactinal series of spines. The spines are 

 well-spaced and proximally the abactinal series are sometimes rather 

 irregular. The larger abactinal plates are three- or four-lobed and 

 are connected proximally by a secondary ossicle (sometimes 2), three- 

 lobed or subtriangular, oblong, or roundish. On the outer part of 

 the ray 3 or 4 smaller, overlapping secondary plates may stand in a 

 longitudinal series between 2 spine-bearing plates which are convex, 

 while the connecting plates are not. The marginal plates form per- 

 fectly regular series. The inferomarginals have 4 rounded corners 

 and are adjacent to the adambulacrals. The first 4 or 5 plates usually 

 each bear a spine; beyond this point every third or fourth plate is 

 spiniferous. The superomarginals are directly above the inferomar- 

 ginals and leave only small skeletal spaces between. Near the base 

 of ray the series turns upward as in other species of Echinaster, 

 leaving a triangular space filled with about 3 series of small inter- 

 marginal plates. On the proximal third of ray usually every third 



