162 bulletin: museum of comparative zoology. 



sephonaster and Tritonaster, and of some species of Astropeeten. 

 The inferomarginals encroach more on to the actinal than do the 

 superomarginals on to the abactinal surface. The proximal margi- 

 nals have narrow shallow fascioles between them, but beyond the 

 third plate they become rudimentary. 



The actinal intermediate plates are in three series parallel to furrow. 

 The first extends nearly to the fifth supermarginal, the second series 

 to the middle of the second, and the third does not reach the second 

 inferomarginal. The second and third series each have an odd 

 interradial while another odd plate, opposite the suture between the 

 first inferomarginals represents the beginning of the fourth series. 

 These plates are dotted Plate 2, figure 5. The first actinal series, that 

 adjacent to the adambulacral plates, does not possess an odd plate. 

 At first sight these odd interradial actinal plates seem to constitute 

 a trivial character, but they are, on the contrary, of generic importance 

 being absent in the following genera: Persephonaster, Tritonaster, 

 Psilaster, Bathybiaster, Thrissacanthias, Astropeeten, Plutonaster, 

 Dytaster, and probably Moiraster. They are present in Leptychaster 

 (arcticus, pacificus, anomalus, propinquus), Patagiaster, Dipsacaster, 

 Tethyaster, and Ctenophoraster. The actinal plates are convex 

 and covered with short slender spinelets. As Verrill states (loc. cit.), 

 the interspaces do not form well developed fascioles, which would 

 hardly be expected since the marginal fascioles are so poorly developed. 

 Internally, the plates are subcircular or ovate with an undulating- 

 outline, and imbricate strongly. 



The adambulacral plates are set somewhat obliquely. They are 

 proximally wider than long, but distally the two dimensions are nearly 

 equal. The first ten plates equal four inferomarginals. The furrow 

 margin is angular, the adoral facet being shorter than the other. The 

 first plate is not conspicuously compressed. The type of armature 

 most nearly resembles that of Leptychaster, and is shown by the 

 figures (Plate 2, figs. 1 and 2). There are usually four or five furrow 

 and numerous subambulacral spinelets, of which none are conspicu- 

 ously enlarged. 



The combined mouth plates are rather broad and convex, the surface 

 being covered with numerous spinelets which are small on the outer 

 parts, but which increase in length toward the inner angle. Marginal 

 spinelets are about nine, tapering, cylindrical, the innermost abruptly 

 enlarged to form a blunt tooth. These two teeth are however ob- 

 scured by the tuft of suboral spines which project over them. 



Spiniform pedicellariae with four to six papilliform blades occur 



