ASTEBOIDEA OF NORTH PACIFIC AND ADJACENT WATERS — FISHEB. 147 



not a fully grown example, and thereff)re has less extended papular areas on ravs, 

 smaller disk, and fewer accessory ahactinal s[)inelets than two examples from station 

 2919. A third and smaller specimen from station •_'919, has a disk relatively as small 

 as that of type and even less extensive papular areas. The slight differences in 

 abactinal spinulation are not sufficient to warrant specific separation. 



In his key, Ludwig (1910) places B. pectinifer in the second section and acan- 

 tlionotus in the first. Acanthonotus belongs in the second section, however, as 

 there is no sharp differentiation between disk and ray in the dorsal spinulation, 

 except \vith respect to the central, and primary basal spines. In this second section, 

 acanthonotus differs from liuddhstoni in having two inferomarginal spines, four or 

 five ambulacral furrow spines, four or live oral spines, two subambulacral spines, 

 and in lacking inferomarginal pedicellariie througlutut the ray. From pectinifer, 

 acanthonotus differs in the rather stouter abactinal spines, somewhat smaller mar- 

 ginal pedicellarise, and single inferomarginal spine; pectinifer differs slightly in hav- 

 ing six to eight oral spines, and five or six furrow spines (instead of four or five) 

 and two or three subambulacral spines (instead of two). The two species are verv 

 close; it is possible they are varieties of one wide ranging form, since acanthonotus 

 shows considerable individual variation. 



BENTHOPECTEN MUTABH-IS Fisher. 

 PI. 22, fig. 1; pi. 23, fig. 3; pi. 54, figs. 6, 6a. 

 Benthopectcn mutabilis Fisher, Zool. .A,nz., vol. 3.5, March 29, 1910, p. .548. 



Diagnosis. — In general appearance similar to B. spinosus Verrill, but ii.sually 

 lacking some of the odd interradial marginal plates (always in one or the other 

 series, as a rule in both); abactinal, actinal intermediate, and inferomarginal 

 pedicellarise, the latter series extending to tip of raj', the first extending far 

 along ray, as a rule; (pedicellarise lacking in B. sjnnosus). Abactinal surface 

 wth several large spines near center of disk, each usually with several small 

 spinelets surrounding; other plates of thsk wath one to three or four very short 

 stubby spinelets, and plates or ray with tj'pically one such spinelet. Supero- 

 marginal with one, inferomarginal ^vith two unequal spines. Adambulacrals 

 with five or six subequal furrow spinelets and two actinal spinelets. R=100 

 mm.; r=15 ram.; R = 6.6 r. Breadth of ray at base, 17 mm. Interbracliial 

 arcs wide and rounded antl beyond papular region the rays slope very gradually 

 to an attenuate extremity; from interradial line to about fifth superomarginal 

 they slope abruptly making the wide interbracliial angles. 



Description. — There are no prominent abactinal spines except near center 

 of disk within the ratlins of the madreporic body. Those spines are about 6 or 7 

 mm. long, tapering and acicular, and are borne on the primary plates: central, 

 radials, basals, and probably infrabasals also in some cases. The other plates 

 of disk bear one to three or four, less commonly five to six or seven, very short 

 stubby spinelets and several usually occur around the base of the central spines. 

 On the rays each plate has usually one central spinelet, but in one specimen there 

 are regularly two, three, or even four, one being larger than the others. This 

 is aberrant, however. These spinelets are all under 1 mm. in length, and most 



