Crustacea.] SUB ANTARCTIC ISLANDS OF NEW ZEALAND. 659 



The specimen from the lagoon is quite similar in colour and other external 

 features to specimens taken at Tomahawk Lagoon and other places in the South 

 Island of New Zealand where the water, though usually fresh, may at times be 

 more or less brackish. The others, gathered in Campbell Island in fresh-water 

 streams at a considerable elevation above sea-level, are rather lighter in colour, 

 and differ slightly in the size of the eyes and in a few other points ; though these 

 differences are in the same direction, they are not so great as those exhibited by 

 the specimens taken in mountain-streams around Dunedin, which I have distinguished 

 as variety y8. This variety differs in the following points from the typical form, 

 variety a, found at sea-level: (1) The eyes are much smaller, being only about half 

 as large ; (2) the front margin of the head has a slight depression in the centre ; 

 (3) the inner antennae are rather more slender and longer, reaching to the end of 

 the third joint of the peduncle of the outer antennae ; (4) the outer antenna is more 

 slender, both in the peduncle and in the flagellum ; (5) there is only one pair of 

 sutures on the terminal segment of the pleon ; (6) the extremity of the pleon is more 

 narrowed ; (7) the colour is usually lighter, being a light brown, with darker spots 

 and markings. 



I am well aware that the numerous small differences between these two varieties 

 are quite sufficient to warrant one in making two species of them ; but from the con- 

 ditions under which the two forms are found it is perfectly clear that they are very 

 closely related, one form having been derived from the other probably quite recently, 

 and there is little doubt that the differences between them are associated with the 

 different conditions under which they live — one in brackish water on the sea-shore, 

 and the other in mountain-streams at a considerable elevation above the sea. If 

 a separate species were made of the latter form its connection with the first one 

 would be in danger of being overlooked ; and, moreover, as I bave already pointed 

 out, the fresh-water forms from Campbell Island, though they are similar to those 

 from the fresh-water streams of New Zealand, are not quite identical, and would pro- 

 bably require to be also distinguished by a different specific name. In New Zealand 

 the species is common in the southern part of the South Island and the adjacent 

 islands. It was originally taken by Mr. Thomson in Tomahawk Lagoon, near 

 Dunedin ; subsequently I took it at the mouth of a small stream running into Otago 

 Harbour, and found the variety /3 in various mountain-streams around Port Chalmers 

 and Dunedin. I have also specimens of variety a in my collection from Ruapuke 

 Island (collected by Mr. T. Horan), and from Port Pegasus, Stewart Island (Dr. L. 

 Cockayne), and more recently I found it in the West Coast Sounds of Otago at the 

 mouth of nearly every fresh-water stream that I was able to examine. I have not 

 succeeded in finding it at Lyttelton or on Banks Peninsula, though I have examined 

 many likely places in these localities. 



A form which apparently belongs to this species is found at Port Henry, Straits 

 of Magellan, and is represented by specimens in the British Museum, which were 

 referred to I. lacustris by Miers in 188L Miers gives one or two 'small points in 

 which these specimens differ from Thomson's original description, but these are not 

 so great as the differences between varieties a and ^. 



Since the above remarks were written I have sent specimens of both varieties 

 to Dr. W. T. Caiman, of the British Museum, who has kindly compared them with 

 the specimens in the Museum collections from the Straits of Magellan, and he informs 



