86 BULLETIN 76, UNITED STATES NATIONAL MUSEUM 



Anatomical notes. — The gonads, which are small with large eggs, open just above 

 the third superomarginal (in the single case observed). Each ovary is subdivided 

 into five lobes. 



Type.— Cat. No. E. 1425, U.S.N.M. 



Type-locality. — Station 4427 7 miles southwest of Point San Pedro, Santa Cruz 

 Island, Calif.; 447-510 fathoms, black mud, rocks. 



Remarks. — This species may be distinguished from Jiyperoncus Clark by the form 

 of the straight pedicellariao. In Jiyperoncus these are never broadly spatulate, but, 

 on the contrary, are much compressed, lanceolate in profile, and the ends of the jaws 

 are hooked. Furthermore, in hyperoncus the tube-feet are biserial and the ambulacral 

 pores are broadly elliptical, less slit-like. 



Chiroplus differs from atactics in the form of the crossed pedicellariae, in having 

 a more lax, open, abactinal skeleton, and a broader intermarginal area, with larger 

 skeletal meshes, and proximally intermarginal secondary ossicles. Atactus has a 

 rather rigid skeleton, that of chiroplus is flexible; atactus has on the distal plates 

 usually two adambulacral spines; chiroplus never more than one. The madreporite 

 of atactus is relatively larger than in chiroplus. 



The difference in the crossed pedicellariae is quite striking, and as these are 

 generally very conservative as regards their form, I think they are of considerable 

 value for identification. A comparison of figures will show that the crossed pedicel- 

 lariae of hyperoncus and atactus are more nearly alike. The straight pedicellariae of 

 chiroplus appear very large, as they are actually as big as in the very much larger 

 specimen of atactus. Whether they increase further in size is not known. 



AMPHEKASTER ATACTUS, new species 



Plate 31, Figure 4; Plate 32, Figures 3, 3a-36; Plate 38, Figure 5 



Diagnosis. — -Rays five. R 60 to 65 mm.; r 9 mm.; breadth of ray at base, 

 10 mm.; R = 6.6 to 7.1 r. Rays stout, slightly inflated, tapered from broadest part 

 just beyond base; superomarginals actinolateral in position; intermarginal channel 

 narrow. Spinelets short, conical, rough, rigid, uniformly rather widely spaced; 

 inferomarginal spines about as long as and a little stouter than adambulacral spines, 

 which stand one to a plate except near end of ray where there are two ; straight pedi- 

 cellariae, large, broadly lanceolate, with three or four strong claws to each jaw. Dif- 

 fering from A. marianus in having minutely thorny spines, smaller marginal spines, 

 narrower intermarginal channel, mostly three-lobed superomarginals, diplacanthid 

 distal adambulacrals, and larger and differently formed straight pedicellariae. 



Description. — The skeleton of this species differs from that of marianus in having 

 a broader dorsolateral region, with fewer, larger, and more irregular openings, a 

 less regular and less prominent carinal series, narrower intermarginal region, with 

 less regular, generally three-lobed superomarginal plates. The carinals have very short 

 lobes and proximally the arrangement is very irregular. It is rather difficult to 

 analyze the dorsolateral skeleton precisely. Distally, on each side of the carinal 

 plates there are two series of broad meshes separated by a longiseries of Y and A. 

 shaped plates, which becomes more and more irregular toward base of ray. On the 

 proximal half or two-thirds of ray the region between this primary dorsolateral series 

 and the superomarginals widens much more rapidly than the adradial band of meshes. 



