Male considerably smaller than female and of a rather different external 

 appearance. Cephalic segment transversely truncated in front, with the lateral 

 corners sub-angular; rostrum almost obsolete. 3rd free segment of metasome 

 with well-developed epimeral plates of the same appearance as those of the 2 pre- 

 ceding segments. Genital segment but slightly expanded, and not encompassing 

 laterally the distal part of the urosome. Caudal rami much shorter than in fe- 

 male, being considerably broader than they are long. Anterior antennae very 

 strongly built and angularly bent at the base, 4-articulate, 3rd joint considerably 

 dilated, last joint unguiform and very mobile. Last pair of legs spatulate, inter- 

 calated between the epimeral plates of the 3rd free segment of metasome and 

 the urosome, tip obliquely truncated and armed 'with a row of 6 short spines. 



Body in both sexes of a clear yellowish green colour, and generally 

 provided dorsally with a dark violet transverse band across the posterior part 

 of the cephalic segment, urosome in female tinged at the base with a similar colour. 



Length of adult female 0.90 min., of male 0.60 mm. 



Remarks. The remarkable sexual difference occurring in this form has 

 led Prof. Brady, and perhaps also Glaus, to describe the two sexes as 2 different 

 species. There cannot, in my opinion, be any doubt that the form recorded 

 by Prof. Brady as P. viride, and regarded as identical with P. dentatum of Claus, 

 is nothing but the male of the present species. True, a figure is also given of 

 the supposed female of P. viride; but the specimen drawn is certainly not adult, 

 and appears to be a young male, in which the anterior antennse have not yet 

 been fully transformed. Nor can I doubt that the P. subrotundum of Norman 

 is founded upon immature specimens of the present species (compare the figure 

 of such a specimen given on the accompanying plate). The PorceUidi'um fasciatnm 

 of Boeck is not, as opined by Prof. Brady, identical with Aspidiscus fasciatus 

 Norman, but is most certainly the present species. Whether Philippi's species is 

 the male of this species or a distinct form, it is very difficult to decide. The female 

 of the present species, at any rate, is easily recognizable by the greatly expanded 

 genital segment and by the characteristic form of the caudal rami. 



Occurrence. I have met with this form rather frequently in several places 

 on the west coast of Norway, as also in the Trondhjem Fjord. It lives, as a rule, 

 on the fronds of Laminarise, to which it applies its flattened body so closely, 

 that it is only with great difficulty that it can be loosened from its hold, when 

 alive. On employing its natatory legs, the animal runs about rather quickly along 

 the fronds or other smooth objects, with an even, gliding motion. 



Distribution. British Isles (Brady), Mediterranean (Claus), Ceylon 

 (A. Scott). 



