380 BULLETIN 16, UNITED STATES NATIONAL MUSEUM. 



which it differs in having reduced median radial paxilliB (not mentioned, liowever, 

 in Sladen's description), more often two than three adambuhicral spinelets, more 

 numerous spiracula, and differently formed mouth spines. 



Of the species described by Ludwig from the Panama-Galapagos region, tlie only 

 one with which the present needs comparison is II. gracilis, from which -perissonotus 

 differs in the great disparity in size between radial and marginal paxillae, in having 

 three or four, rarely live, paxilla spinelets (not as many as seven), in having the abac- 

 tinal plates (radial and adradial) with only one to three basal processes, not four. 

 The abactinal plates of gracilis are regularly disposed forming cjuadrate meshes, and 

 the superomarginal plates have five processes, not two or three as in perissonotus. 

 The abactinal integument of perissonotus contains calcareous platelets in the meshes 

 of the skeleton; gracilis has none. Gracilis has three or four oral spinelets, perisso- 

 notus has five to seven. The madreporic body of gracilis is small, that of perissonotus 

 very large. 



This species is readily distinguished from others described in this report by the 

 remarkable structure of the abactinal paxillse which with their supporting base are 

 greatly reduced on the radial areas, the marginal paxillse being very much larger. The 

 nidamental cavity is chief!}' between the rays, in consequence of the smallness of the 

 radial paxillse. The number of adambulacral spines is usually two but sometimes 

 three. 



HYMENASTER QUADRISPINOSUS Fisher. 



PI. 108, fig. 2; pi. 110, figs. 1-3; pi. 117, fig. 2. 



Eymenaster quadnspinosus Fisher, Bull. Bur. Fisheries for 1904, vol. 24, June 10, 190.5, p. 31.5; 

 Ann. and Mag. Nat. Hi.st., ser. 8, vol. 5, Feb., 1910, p. 170. 



Diagnosis. — Adambulacral spines four or five; aperture papilla broad, some- 

 times with a sacculus at tip; paxillar spines three or four (rarely five), the paxiDae 

 being arranged in nine rows along ray and forming speciaUzed areas; lateral web 

 when not contracted extending nearly to tip of ray, with transverse parallel linear 

 spiracular areas; spiracula variable with age, very numerous, small, not in definite 

 areas but everj'where among the spine tips; paxilla crowns of adjacent rows alter- 

 nating. 



Description. — The specimens exhibit extreme contractiUty of the interbrachial 

 web and variability of contour and proportions. In most of the specimens this 

 muscular interbrachial membrane is quite contracted, the paxillar crowns extending 

 to edge of body, wliich has a very stellate form. In two specimens, however, the 

 membrane is not contracted and exhibits transverse parallel lines of spiracula as 

 described by Sladen in //. nobilis, and the form is more pentagonal with the paxillar 

 areas sharply differentiated. The largest specimen has the rays too much recurved 

 to measure. A slightly smaller one with contracted interbrachial webs has R=51 

 mm., r = 27 mm., R = 1.88 r; a specimen with partly expanded web measures, 

 R=42 mm., r = 29 mm., R = 1.48 r. 



Supradorsal membrane thin but tough, of a satiny luster, containing very 

 numerous but inconspicuous crisscrossing muscle fibers (\-i.sible only in the larger 

 Alaskan examples). Paxillar area comprising whole dorsal surface, except a free 

 lateral muscular web wliich is usually so contracted as to be very inconspicuous. 



