the eye-stalks, and the rostrum slender and moderately 

 long as in some allied forms of the genus, while some of 

 the Pacific specimens of both sexes have the frontal plate 

 with rostrum developed fully or even more anomalously 

 than in the specimen figured by Ortmann, and some other 

 specimens show connecting links between the normal and 

 the aberrant shape. These intermediate stages together 

 with the fact, that the specimens with the frontal plate 

 greatly expanded agree in every other feature — the struc- 

 ture of the male copulatory organs included — with the 

 specimens possessing a normal frontal plate with slender 

 rostrum, prove with absolute certainty that all specimens 

 belong to the same species. The sketches convey better 

 than any explanation an idea of the very different aspect of 

 the two « forms » of this species. 



Fig. i. Euphausia diomedeœ Ortm. 

 Anterior part of a normal male 

 from the Indian Archipelago. 



Fig. 2. Euphausia diomedeœ Ortm. 

 Anterior part of a male with 

 the frontal plate expanded, East 

 Pacific. 



And this variation in Euphausia diomedeœ Ortm. is 

 not without parallel within the same genus. In E. triacan- 

 tha Holt and Tatt., of which numerous adult specimens 

 are to hand, the frontal plate is rather short, covering 

 only the base of the eye-stalks, and produced into a rather 

 long or long rostrum. But in an adult female the plate is 



(210) 



