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



but having the fourth somite of the pleon, and sometimes also the third, carinated in the 

 adult ; and the telson is laterally armed with six spinules. This corresponds with the 

 normal condition of Crangon affinis, as well as with the European type. The slight 

 variations between the two forms, though constant, do not appear of sufficient importance 

 to lead me to consider them as specifically distinct, and it is doubtful if the Japanese 

 specimens can be considered to possess features that are sufficiently important to warrant 

 specific distinction. 



The late Professor Kinahan, 1 however, considered the channelling of the dorsal surface 

 of the posterior somites of the pleon so important that he founded the genus Steira- 

 crangon on this feature alone. That it may be sufficiently marked in some forms as to 

 be of specific value is probable, but it is so slight in the Japanese specimens that it is 

 only appreciable when carefully examined, and I do not think we are justified in con- 

 sidering it as more than a variation in form from the normal European species. De 

 Haan states that Crangon affinis possesses the fluted telson, and that the lateral spines 

 of the carapace are larger than the median. But this is scarcely the case in our speci- 

 mens, since in well-formed animals the spines are equally well developed. 



The only distinction that is at all appreciable exists in the different lengths of the 

 telson, but this difference is too slight to warrant its recognition as a specific character ; 

 it rather demonstrates the line of departure under certain conditions in which variation 

 may proceed. Two of the specimens from Yokoska that I attribute to Crangon vulgaris 

 were taken in somewhat shallower water, and approximate nearer to the European form. 

 But since de Haan, Stimpson, and Kinahan have thought the channelling of the dorsal 

 surface of the telson to be sufficiently important to be of sjjecific value, and as the 

 Japanese form has been distinguished by a specific name, I have thought it preferable to 

 retain de Haan's name " Crangon affinis," which I have no doubt is also synonymous 

 with Crangon propinquus, Stimpson. 



Pontophilus, Leach. 

 Pontophilus, Leach, Malacos. Decap. Brit., Tab. ix. 



Like Crangon, but has the second pair of pereiopoda short and chelate, the third long 

 and styliform. There are seven pairs of branchiae, including a small podobranchial plume 

 attached to the first gnathopod. The branchial arrangement may be tabulated as 

 follows : — 



Pleurobranchife, . . . ... 1 11111 



Arthrobranchiae, . . . 



Podobrancbise, . 1 



Mastigobranchise, . . 1 r 



hi k 1 m n o 



1 Loc. cit., p. 58. 



