54 Proceedings of the Royal Physical Society. 



lochs, while D. laticeps takes its place in those at a lower 

 level. Elsewhere it is found in lochs very little above sea- 

 level, such as Loch Lomond and Loch Shiel, but those lochs 

 are always in mountainous districts, while D. laticeps and 

 D. luierzejskii are often found in lowland districts. 



Diaptomits laticeps, G. 0. Sars, is very closely related to 

 the following species, D. luierzejskii. The most conspicuous 

 difference between them is found in the process on the 

 antepenultimate joint of the right antenna of the male, 

 which is pectinate in D. luierzejskii, plain in D. laticeps. 

 In certain lochs of Sutherland, various gradations between 

 the two forms of process have been observed, along with the 

 typical forms of both species, and I regard D. hircus, Brady, 

 as such an intermediate form ; nevertheless, I have in this 

 study of distribution kept the two species apart, in deference 

 to the opinion of Mr Scourfield and other authorities who 

 consider them to be good species, and because, although the 

 differences between them may be of little systematic im- 

 portance, they must have some physiological significance, 

 since their distribution is widely different, D. wierzejskii 

 being a purely northern form, while D. laticeps has an 

 extensive range on the west. D. laticeps is the commonest 

 species of the genus, after D. gracilis, being found in 

 Shetland, Sutherland, Eoss, Perth, and Argyle, common in 

 the Outer Hebrides and Inverness, and having its eastern 

 limit in some small lochs of Perthshire, very close to those 

 Aberdeen lochs in which Mr Clark found D. laciniatus. 

 The area of distribution of this species in Scotland corre- 

 sponds approximately with that of D. laciniatus, but in 

 spite of this general agreement, the two species are almost 

 completely separated : though they often occupy adjacent 

 lochs, I know of only a few lochs in which both species 

 occur together. 



D. wierzejskii, Eichard, is very abundant in Shetland, 

 common in the Outer Hebrides, and occurs in a few lochs 

 in Sutherland, and one loch in Eoss. I have recorded it 

 as occurring in some hill lochs of central Perth, but those 

 records really refer to D. laticeps, which at that time I 

 united with D. wierzejskii as a mere form. 



