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[Read before the Philosophical Institute of Canterbury, 2nd December, 1914.]
In “Das Tierreich” Amphipoda Gammaridea Mr. Stebbing describes nine accepted species of the genus Elasmopus and four doubtful ones. Of these, two are recorded as having been found in New Zealand seas—viz., E. subcarinatus (Haswell) and E. viridis (Haswell)—both of which were also known from Australia, and were originally described in 1879 from Australian specimens. In the case of the first species, which had been redescribed by Mr. G. M. Thomson in 1882 under the name of Maera petriei, I pointed out many years ago that there were two forms of male apparently both belonging to this species, but no fuller investigation of the matter has yet been made, although in the meantime the species has been recorded from the shores of Ceylon and other places in the Indian Ocean. In the endeavour to work out an Australian species of Elasmopus I have been led to look into the New Zealand species, and the following notes are the result.
The genus appears to be closely allied to Maera, and in some cases it will probably be difficult to decide in which of the two genera a particular species should be placed. The species E. subcarinatus and those allied to it appear to be distinguishable from Maera by the small accessory flagellum, the robust peraeopoda, and by the third uropod not reaching far beyond the others.
I have not included Maera viridis (Haswell), which Stebbing has placed under Elasmopus, as it differs in several respects from the other species, and appears to me to be very close to Maera inaequipes (A. Costa) and best left under Maera.
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So far as the New Zealand forms are concerned, the characters which seem most useful for distinguishing the species are whether the pleon is carinated or not, the character of the carination when present, and the shape of the second gnathopods in the male. The second gnathopoda are usually much more largely developed in the males than in the females, and their distinctive characters appear to be attained only in fully developed males, so that the particular form of gnathopoda may vary considerably owing to the age and development of the animal; and from what has been stated below it seems probable that in one species there are two forms of gnathopoda in the adult males, apparently similar to what has been described by Mrs. E. W. Sexton in the case of Jassa falcata (Montagu). In these cases it is, of course, difficult to decide whether we are dealing with one species with two forms of male, or with two different species in which the females are practically indistinguishable although the males are different. In the following list I have treated the forms as belonging to separate species, except in the one case where the differences between the males are confined to the second gnathopoda.