PYCNOGONIDA DREDGED DURING THE CRUISE OF H.M.S. "TRITON." 9 



in this species. If the genus Pallenopsis be accepted, four other species of 

 deep-sea Pycnogonids, collected during the voyage of H.M.S. "Challenger," 

 and described in my report on the Pycnogonids as belonging to Phoxichilidium, 

 Milne Edw., belong doubtless to it also. So we have eight species of this 

 genus, the range of depth and geographical distribution of which may be judged 

 from the following list : — 



Name of the Species. 



Depth in Fathoms. 



Geographical Distribution. 



Pallenopsis flitminensis, Kroyer, spec, 



7-20 



Patagonia, Brazil. 



„ patagonica, Hoek, spec, 



45-175 



Patagonia. 



„ forficifer, Wilson, 



262-333 



Off South Carolina. 



„ longirostris, Wilson, 



500 



South of Cape Cod. 



„ tritonis, n. sp., 



516 



Faroe Channel. 



„ pilosa, Hoek; spec, 



1600-1950 



Southern Indian Ocean. 



„ oscitans, Hoek, spec, 



1675 



Atlantic, West of Azores. 



„ mollissima, Hoek, spec, 



1875 



Off Yeddo (Japan). 



Of these, six are true deep-sea species, the two others are shallow-water 

 inhabitants. The deep-sea species have the mandibles three-jointed (as are those 

 of the young of Colossendeis and Ascorhynchus) ; the two shallow- water species 

 show a transitory form between those with three and those with two-jointed 

 mandibles. In the mandible of P. patagonica a trace of an articulation is 

 visible dorsally, but totally wanting when seen from the ventral side ; in P. 

 fluminensis, a little beyond the middle, the basal joint of the mandibles is 

 furnished with a row of hairs, and seems to be divided into two. 



Within the limits of the genus Pattenopsis the change of three-jointed into 

 two-jointecl mandibles has taken place. That the three-jointed mandible must 

 be considered as the original form is shown by the mandibles of different 

 species of Ascorhynchus and Colossendeis ; though they exist in these genera as 

 rudimentary or larval organs only and are too small and too weak to be of use to 

 the animal, they are distinctly three-jointed. Larval parts, or parts which have 

 grown rudimentary, are no more strongly influenced by circumstances ; 

 hence they often retain their original condition. No doubt it is a very 

 curious coincidence, that the deep-sea species show the original condition of 

 the mandibles, whereas the shallow-water forms are furnished with these 

 organs in the more robust condition of most of the other genera of Pycnogonids. 



Finally, I wish to point out that the new species for which I have proposed 

 the name P. tritonis comes very near to P. longirostris, Wilson. I was long uncer- 

 tain whether I should refer my specimen to that species or should describe it as 

 a new one. I chose the latter, because of numerous, though perhaps not 

 very important, differences between his description and my specimen. Should 



VOL. XXXII. PART I. B 



