71 



specimens were taken, and in examining these I have found 

 that there is very considerable variation in many of the characters 

 on which Milne-Edwards and Bouvier base their distinctions. 

 For instance, the spines on the sides of the rostrum vary in length 

 and sharpness, and in those most nearly approaching the nexa 

 type they are not blunter or thicker than in many of the dispersa 

 type. The rostrum also varies in length, and it is often impossible 

 to say whether it " almost reaches the extremity of the antennu- 

 lar peduncle " or " just passes the base of the last joint." It 

 depends very much on the way in which the antennules are 

 extended. Another character given by Milne-Edwards and 

 Bouvier concerns the teeth or spines on the transverse furrow 

 immediately behind the rostrum. They say that in dispersa 

 "il y a aumoins deux paires de saillies spiniformes ou d'epines," 

 and in nexa "il n'y a pas de saillies, ou seulement une paire de 

 saillies a peines distinctes," yet in many of the Helga specimens 

 of undoubted dispersa form there are no teeth or spines on this 

 line, and in others only a single pair. Again, in the great majority 

 of the Helga specimens of the dispersa type the lateral margins 

 of the carapace are distinctly convex, not " sensiblement droits.'* 



The spines on the third maxillipedes show considerable 

 variation. In some cases the large spine is followed by three 

 or four smaller ones, in others by only one. In two large males 

 from the west coast there is only one very small spine distal 

 to the large one. These two specimens approach more nearly 

 the nexa type than any of the others, and this lends support 

 to Hansen's view that nexa has been founded on large male 

 specimens of dispersa, Bonnier's description of nexa was taken 

 from a single large male, and Milne-Edwards and Bouvier also 

 saw only a single specimen, a male, whereas Appellof and Hansen 

 both had a large amount of material. 



More recently Crawshay (1912) has separated the two forms 

 by the character of a small group of three or four setae on the 

 third maxillipedes close to the base of the large meral spine. 

 In nexa these are simple, and in dispersa pinnate. They are 

 pinnate in all the Helga specimens except the two large males 

 mentioned before (from. Ballynakill Harbour, Co. Galway), in 

 which they are simple. These two also possess the strong spnia- 

 tion and hispidation of the chelipeds of which Crawshay speaks, 

 but the third maxillipedes bear a spine distal to the large central 

 one, so that the characters of nexa and dispersa are here to some 

 extent combined. 



I am quite convinced, with Hansen, that the species nexa 

 has been erroneously founded on very large male specimens of 

 dispersa. The dispersa forms are very much commoner than 

 the other, but the name nexa has priority and so must be used 

 for the united species. 



Size. — The largest specimen in the Irish collection is a male 

 measuring 40 mm. 



General Distribution. — The distribution of this is practically 



