228 BULLETIN 76, UNITED STATES NATIONAL MUSEUM. 



LDIDIASTER DAWSONI (VerrUl). 

 PI. 25, fig. 2; pi. 26, fig. 3; pi. 27, fig. 2; pi. 55, figs. 3, 3a; pi. 5(!, fig. 5; ])1. 1 19, fig. 2; pi. 120. 



Archasler dawsoni Verrill, in Appendix C, On Some Marine Invertebrata from the Queen 

 Charlotte Islands, By J. F. WTiiteaves, Report of Progrea.^; Geol. Survey of Canada for 

 1878-79, 1880, p. 194fc. 



Acantharchaster daiisotti, Verrii.l, Proc. U. S. Nat. Mus., vol. 17, 1894, p. 269.— Fisher, Zool. 

 Anz., vol. 35, March 29, 1910, p. 549. 



Luidiaster dawsoni LvDwta, Sitzungsber. k. preus.-^. Akad. Wlss., vol. 23, 1910, p. 452. 



Diagnosis. — Rays five. R = 124 mm.; r = 22 mm.; R = 5.45 r. Breadth of 

 my at base, 25 mm. A narrower-rayed example: R = 107 mm.; r = 21 mm.; 

 R = 5.1 r. Breadth of ray at base, 25 mm. In the first specimen the rays taper 

 evenly to the tip; in the second, abruptly at first, then very gradually. Inter- 

 brachial arcs of first specimen, angular; of the second, distinctly rounded. Disk 

 small. Abactinal surface paved with small unequal plates in the form of low simple 

 paxilla?; the larger of these plates bearing a large central articulated spine sur- 

 rounded at base by a circle of small spinules (usually) ;, the smaller paxillre without 

 central spine and with comparatively- few divergent papilliform spinelets. Papulae 

 confined to disk (except center and interradial lines) and basal part of rays. Double 

 or triple pectinate peilicellariaj of conspicuous size on dorsal surface, and one to 

 three still larger in each interradius of actiual surface. Each pedicellaria with 

 upward to twelve curved papillse or spines to each group. Anus present. Mar- 

 ginal plates con.spicuous, alternate, those of the upper series smaller than the lower, 

 rounded, with a central eminence bearing one or two large bristling movable spines 

 surrounded by spinules. Inferomarginals with about three large spines. Adam- 

 bulacral plates with a salient furrow angle bearing a divergent group of furrow 

 spines (about five) and a transverse actinal row of two to four long slender blunt 

 spines. Actinal intermediate plates very few and confined to interradial region. 



Description. — Abactinal area plane, or slightly swollen at base of rays. Abacti- 

 nal surface paved with larger round plates interspersed with smaller more irregular 

 ones. Viewed from the coelomic surface these plates are seen to be irregularly 

 arranged and close together, but not in actual contact. Papulae are absent from 

 center of disk, a narrow interradial line, and distal two-thirds of R. They are pres- 

 ent, therefore, on basal part of ray and adjacent portion of disk. On this area there 

 is some semblance to a formation of longitudinal rows by the plates of the medio- 

 radial region, and here also there is a greater difference between the two sizes of 

 plates. The smaller plates act as connectives between the larger ones, and the large 

 papidffl pass through the interspaces. The plates form low, simple paxilla? (proto- 

 paxilla"). The tabulum is only very shghtly elevated. The large paxillje, corre- 

 sponding to the large plates, each bear a long, shaip, movable spine (sometimes two), 

 encircled by several shorter unequal spinules (two to six or eight). On the edge of 

 the tabulum is a circle of numerous small spinelets. Sometimes the auxiliary spi- 

 nules are very few or lacking and tlieir place is taken by small spinelets. The smaller 

 paxilla; l)ear one to twelve small, slender (or thicker, papilliform), acute spinelets. 

 Tlie dorsal spines are largest in center of disk, being there about 5 mm. long and 

 fairly stout. Beyond the papular area they decrease markedly in size, as may be 



