132 BULLETIN 76, UNITED STATES NATIONAL MUSEUM. 



Horridus. Dawsoni.'^ 



Abactinal paxiUie of dwk frequently with accos- Abactinal paxilla; without elongated accessory 



sory spinuk'H one-half as long as spine. spinules. 



Madreporic plate situated its own diameter dis- Madreporic plate nearly touching superomarginals 



tant from superomarginals. (in young specimens only). 



No pedicellariffi. -^ •''■'Se pectinate pedicellana on most or all of the 



actinal interradial areas. 6 

 Mouth plates with three slightly enlarged actinal Mouth plates with about four very prominent ac- 

 spLuuIes. '^'"''^ spines (as long as the proximal subambu- 



lacral spines) parallel to furrow series. 



Luidiaster oxyacanthus (Sladen) differs from dawsoni in the following particu- 

 lars: in dawsoni the paxillte bearing special central spines extend to end of ray, 

 only onto base in oxyacanthus; dawsoni frequently has two superomarginal spines, 

 rather than only one; furrow spinelets longer, slenderer, and the series with a 

 longer base line than in oxyacanthus; the subambulacral spines longer and fre- 

 quently three (nearly as long as the inferomarginal s]nnes); actinal intermediate 

 pediceilaria? larger, each comb having eight to twelve spinelets (about five in 

 oxyacanthus); mouth plates with more abruptly enlarged inner teeth and seven to 

 ten spines on the actinal surface (about five in oxyacanthus). The disk of oxya- 

 canthus is smaller than in dawsoni and the interbrachial angles more abrupt, less 

 rounded. 



Genus NEARCHASTER Fisher. 



Acantharchaster Fisher (part), Zool. Anz., vol. 35, March 29, 1910, p. 549. 



Nearchaster Fisher, Ann. and Mag. Nat. Hist., ser. 8, vol. 7. Jan., 1911, p. 91. Tjqje, Acanthar- 

 chaster aciculosus Fisher. 



BenthopectinidtE with the odd interradial marginal plate lacking in from 

 one to four interradii, though sometimes present in all; not always present in both 

 series of the same interradius; abactinal plates large and small intermingled, the 

 large primary plates bearing on a low tabulum a long shar]> spine surrounded by 

 a circle of accessory slender spinules, variable as to number and length — sometimes 

 over half as long as the prinaary spine; secondary plates with a group of shorter 

 spinelets; prunary plates of papular areas strongly stellate; papulae not in circum- 

 scribed areas, but distributed all over disk (except sometimes the very center) 

 and along ray from one-third to tlu^ee-fifths its length, being confined to either 

 side of the paxillar area distally; rays very long and slender; marginals extremely 

 spiny — two or tlu-ee long, sharj) superomarginal spines and two to five inferomar- 

 ginals; subambulacral si)ines one to three, usually two, long; furrow spines one to 

 seven; mouth plates large, witli numerous marginal spines, those of each pair of 

 plates disposed in three intlependcnt series: the median teeth which are the largest 

 and fewest, and on either side a series of smaller spines subequal or graduated in 

 length toward the mesial members of the group; pedicellarise when present large, 

 pectinate, on any or all of the following plates: abactinals, inferomarginals, actinal 

 intermediates; dorsal muscle bands not attached to an ambulacral ossicle, rather 

 weak. 



"Taken from immature epecimen; a large specimen would vary slightly from this. The type of 

 horridus is an immature form, in all probability. 



6 Erroneously stated by me to be lacking in young dawsoni. The specimen examined then turns out 

 to be young NearchasUr variabilis. The attachment of the muscle bauds ia unknown in horridus. 



