60 Proceedings of the Royal Physical Society. 



Apart from the species of the genus Diaptomus, the dis- 

 tribution of which is indicated in the preceding Table, the 

 Arctic Crustacea, which are so abundant in the Scottish 

 plankton in summer, are not confined to the western or 

 northern portions of the country, but range over the whole 

 length and breadth of Scotland. 



Although we are here considering only the Scottish 

 plankton, another fact of interest in connection with the 

 geographical distribution of the Arctic species of Crustacea 

 will appear if we extend our view over the whole of Great 

 Britain, as we are enabled to do by Mr Scourfield's tables 

 of distribution, given in his Synopsis (5). 



The whole of Great Britain is characterised by a succes- 

 sive dropping out of the Arctic species of Crustacea as we 

 proceed from north to south. First, Diaptomus wierzejskii 

 ceases in the extreme north, then D. laciniatus in Ayrshire, 

 I), laticeps (if we include with it D. hircus) in Wales. The 

 rest of the Arctic species of Crustacea, which are common 

 over the whole of Scotland, extend also into the north of 

 England, Wales, and in some cases the midlands ; but, with 

 the one exception of DaiJhnia hyalina, they are all absent 

 from the south and east of England. The south and east 

 of England forms, therefore, a well-marked region, dis- 

 tinguished, first of all, by the absence of all the Arctic 

 Entomostraca except Daphnia hyalina.' The demarcation 

 of this region from the rest of England is emphasised by 

 other peculiarities. Many species of Crustacea of a more 

 southern type have the same limitations as those of the 

 Arctic type just referred to, that is to say, they are of 

 general occurrence in Scotland, the north of England, usually 

 Wales and the midlands, but they are absent or are rare in 

 the south and east. In addition to those negative characters 

 marking off this region from the rest of Great Britain, 

 positive characters are found in the presence in this area of 

 three or four species of Entomostraca, which are absent 

 from the north of England and from Scotland, and of several 

 others which are common in the south but rare in the north. 



The features distinguishing this region of England from 

 the rest of Great Britain will be most graphically exhibited 



