10 THE VOYAGE OF H.M.S. CHALLENGER. 



Station 235. Off Japan, south of Omae saki. June 4, 1875. Lab. 34° 7' 0" N., long. 

 138° 0' 0" E. Depth 565 fathoms. Green mud. Bottom temperature 38°-l Fahr.; sur- 

 face temperature 73°"0 Fahr. 



Remarks. — Pararchaster semisquamatus and Pararchaster antarcticus are readily 

 distinguished from the other species of the genus by the absence of pedicellariae and the 

 simplicity of the spinulation of the abactinal plates. The differences between the two 

 forms are discussed in detail in the description of Pararchaster antarcticus. 



If these two species and the variety are viewed together as an independent or spe- 

 cially characterised type of the genus, their distribution is very remarkable and instruc- 

 tive, Pararchaster semisquamatus of the North Pacific being represented by a variety in 

 the North Atlantic, whilst the closely-allied species Pararchaster antarcticus, from the 

 Southern Ocean, presents some of the characters of the typical or Pacific form of Parar- 

 chaster semisquamatus, as well as some of those of its Atlantic variety occidentalis. 



[ la. Pararchaster semisquamatus, var. occidentalis, nov. 



There is a single specimen from the western side of the North Atlantic, off 

 the east coast of North America, which, although agreeing in a remarkable way in all 

 essential points with the type just described, presents a number of variations which 

 render it worthy in my ojjinion of nominal recognition, — in fact, it may ultimately prove 

 to be a distinct species. At present, however, I hesitate from according it that rank on 

 the slender evidence of a solitary and imperfect specimen, although the widely separated 

 geographical position of the two dredging stations would certainly favour the adoption of 

 such a course. 



The two forms are almost exactly of the same size. In the Atlantic example — the 

 variety under notice — the spines on the supero-marginal plates are distinctly thicker and 

 more robust at the base, whilst those on the infero-marginal plates are relatively smaller 

 than in the Pacific form (the type). The two large spines on the actinal surface of the 

 adambulacral plates are also smaller and shorter. On the abactinal surface the single 

 minute thornlet which springs from the centre of the abactinal plates is distinctly shorter 

 and thicker — a circumstance which gives at first sight a finely tuberculate character to 

 the abactinal area when viewed from above. The large spines in the central region of the 

 disk are smaller and much less numerous than in the Japanese form, and do not extend 

 to the base of the rays. The lateral wall at the summit of the interbrachial arc is much 

 less bevelled towards the abactinal surface of the disk, and the marginal plates do not bend 

 over so conspicuously as in the type figured. The infero-marginal plates appear propor- 

 tionately smaller in their transverse dimensions ; and the marginal or furrow series of 

 spines on the adambulacral plates are slightly more delicate and elongate. The roughen- 

 ing of the surface of the large spines on the marginal plates and elsewhere is more 



