106 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



The distribution of the inferomarginal spines together with the 

 presence of enlarged teeth will separate this form from any species 

 classified by Sladen under Psilaster. 



Psilaster gotoi and Ps. rohustus, as well as the following unnamed 

 species belong to Koehler's genus Phidiaster. The characters of 

 these species demonstrate that the structure of the superomarginals 

 varies greatly. The superomarginals may be very narrow dorsally 

 (pectinatus) , or conspicuously broad (agassisi, rohustus), or medium 

 broad (gotoi). They ma}^ have 1 series of spines {gotoi, rohustus, 

 pectinatus), or 2 series (agassisi), or none (the following form). 

 The size and armature does not seem to be of generic value. The 

 inferomarginals vary also from having only 1 complete series to those 

 having 2. In typical Psilaster the spines form usually transverse 

 combs and are sometimes so placed as not to form also longitudinal 

 series along the ray. But I have a specimen of Psilaster florae, a 

 typical Psilaster, in which the spines form both transverse and longi- 

 tudinal series, not all of the latter completed. In Ps. pecthiatus, and 

 exceptionally in rohustus, the proximal plates may have 3 spines in 

 a transverse series, or to put it differently, an intermediate longi- 

 tudinal series is present. These 3 spines approach the condition of 

 typical Psilaster, such as Ps. florae. The difference is one of number 

 only. Ps. pectinatus, in regard to the disposition of the inferomar- 

 ginal spines, presents an intermediate stage between Ps. florae and 

 Ps. agassizi and Ps. gotoi. The presence of 2 closely appressed 

 rows of spines on each mouth plate is common to typical Psilaster 

 and Phidiaster, but in the latter the 4 inner teeth of the combined 

 mouth plates are larger and are directed horizontally into the 

 actinostome, a condition found also in Ps. gracilis (not otherwise 

 like Phidiaster) . If the inner teeth of Psilaster florae are bent down- 

 ward so as to lie horizontally they are seen to be larger than the 

 other mouth spines, although not quite so large as in Phidiaster. But 

 Ps. pectinatus, otherwise like Phidiaster, has the teeth like Psilaster. 

 Even here, therefore, the difference is rather one of degree, for the 

 armature in arrangement is strikingly alike in the two groups. I 

 have merged Phidiaster with Psilaster because the characters sei3arat- 

 ing the genera are either variable or connected by intergrades. The 

 extreme types, however, are easily enough separated, and Phidiaster 

 will make a convenient subgenus whenever one is needed. 



PSILASTER, species. 



Station 5605, Gulf of Tomini, Celebes, 647 fathoms ; 1 specimen. 



This specimen, which has R 14 mm., is nearest Ps. rohustus, but 

 lacks entirely superomarginal spines and lias loss massive marginals 

 and wider paxillar area. The terminal plate and inferomarginal 

 plates and armature closely resemble those of rohustu-^. Furrow 



