348 BULLETIN 76, UNITED STATES NATIONAL MUSEUM. 



PTERASTER TRIGONODON Fisher. 

 PI. 99, figs. 1, 2; pi. 115, fig. 2; pi. 116, figs. 2, 2a. 

 Pteraster trigonodon Fisher, Ann. and Mag. Nat. Hist., ser. 8, vol. 5, Feb., 1910, p. 168. 



Diagnosis. — Rays five. R = 30 mm.; r=16mm.; R = 1.87r. Breadth of ray 

 at base 18 mm. General form stellate with angular interbrachial arcs. Diskmod- 

 eratety high; vertical diameter about two-thirds breadtli of ray. Supradorsal mem- 

 brane thin, without definite reticulations; calcareous deposits abundant, in the form 

 of irregular spiny rods; spiracula numerous, large; pseudopa.xilla? with high 

 petlicels and six divergent spines; five adambulacral and five webbed marginal 

 mouth si)incs; aperture papilla very broad; suboral spine very large, sharp, and 

 three-edged; actinolateral membrane narrow, not defining ambitus. 



Description. — Supradorsal membrane thin and translucent, the minute calca- 

 reous bodies showing under a moderate magnification; no regular reticulations, 

 but fine muscle fibers proceed radially from the tips of the abactinal spines wluch 

 push up the supradorsal membrane, giving the surface a papillose appearance. 

 Spiracula conspicuous, numerous, but not uniformly distributed, although occur- 

 ring everywhere on the abactinal and lateral surfaces. Some of them seem to 

 have been enlarged by the stretcliing of the membrane. Here and there six or 

 eight form a circle around the central spine of a paxilla. Membrane contains a 

 multitude of minute calcareous bodies in the form of very irregular branched spiny 

 rods, fenestrated spiny bodies, or irregular plates with one or two perforations 

 and long spines attached. Each pseudopaxiUa consists of an elevated slender 

 pedicel about 2 mm. high, surmounted by a central spine and a circle of five around 

 it, all shghtly shorter than the pedicel, and forming the slight protuberances on the 

 supradorsal membrane. Osculum large. 



Ambulacra narrow, feet in two rows. Adambulacral plat«s with a curved 

 transverse fan of five spines, the innermost shortest (one-half or two-thirds the 

 longest), thence increasing in length to the outermost, which is usually longest 

 and situated close to the actinolateraj membrane. As in many species of the genus, 

 the inner end of the series curves aborally. There may be four spines, especially 

 far along ray. The web is deeply emarginat« between the spines, and does not 

 extend laterally on the actinolateral membrane. Aperture papilla wider than 

 usual in proportion to length and with a very convex free aboral edge. Aperture 

 at base of outer comb spine and about one-half its length. 



Mouth plates rather narrow, each with a prominent central elevation or boss 

 for the large actinal or suboral spine. Tins is one and one-half to nearly twice 

 as long as the longest marginal spine, sharp, very heavy, and with three wide lateral 

 flanges or edges, so that the spine has three concave faces, appearing like a minia- 

 ture spear-head. Marginal spines five, graduated toward the innermost, which is 

 about as long as the interradial dimension of plate; all are united by membrane, 

 but companion combs are independent. 



Actinolateral membrane narrow, with a narrow free border which does not 

 define margin of ray when viewed from either surface. Actinolateral spines gradu- 

 ally decreasing in length; proximally about twice as long as outer adambulacral 

 spine. 



