45 



bears four small sharp teeth on the proximal part of the joint, 

 while at the distal end there is a much larger single tooth. 

 The outer keel is quite smooth, and on the distal part of the 

 upper keel there is a very faint granulation, but nothing com- 

 parable to the strong row of teeth present in P. vulgaris. Both 

 the upper and outer keels end in sharp points. Near the distal 

 end of the propodite there is on the under side a very minute, 

 sharp tooth directed forwards. 



The combined lengths of carpus and propodite in the jfifth 

 pair of legs are exactly equal to the combined lengths of these 

 joints in the fourth pair. 



The colour of the specimen (in alcohol) is as follows : — The 

 abdomen is pale violet brown, mottled with pale yellow. The 

 same violet brown colour is present on the carapace and extends 

 half-way up the supra-orbital spines, the distal half of which 

 is yellow ; the extreme tip is a translucent golden brown. All 

 the carapace spines are of this pale yellow tint and have trans- 

 lucent points. The sternum, legs, antennules, telson, and 

 uropods are yellow ; the antennae are marked with alternate 

 bands of brown and whitish yellow. 



The single specimen is a male, and measures 154 mm., from 

 rostrum to end of telson. 



I have great pleasure in naming the species in honour of my 

 first instructor in Zoology, Prof. J. Arthur Thomson, of 

 Aberdeen. 



The record of the single specimen is as follows : — 



Helga, 



S. II. 1178—22 V '11. 58 mis. W. J N. of Blackball Head. 

 51° 20' N., 11° SO' W., 212-229 fms., sand. Trawl- 

 One 154 mm. 



As the three species, P. Thomsoni, P. Gilchristi, and P. 

 vulgaris, are closely related it may be of use to give a summary 

 of their characters in tabular form. 



[Table 



