Miscellaneous. 65 



Dore and those of North Germany : — Ceriodaphnia pvlclielJa, Sars ; 

 HyaJodaplinia apkata, Kurz ; Bosmina longirostris, 0. F. Miill. ; 

 ConochiJus volvox, Ehrbg. ; Aniircea cochlearis, Gosse ; A. longi- 

 spina, Kellicott ; Asplanclma helvetica, Imhof. But many other 

 species do not occur in the Aiivergne, while, on the other hand, 

 Holopedium r/ibberum, Zaddach, has not been found in the lakes of 

 North Gennany. A comparison with the various European faunas 

 gives similar results. 



Comparisons thus made are not of great importance, especially for 

 the establishment of regions with distinct pelagic faunas. In the 

 first place it is necessary to make continuous and methodical inves- 

 tigations at different periods of the year. The European lakes will 

 then, for the most part, present a multitude of common species, 

 transported from the north of Europe, their centre of dispersion, 

 from lake to lake, in the state of winter-eggs, by birds or by tlie 

 winds. It is only by passive migrations that we can explain the 

 existence of the pelagic fauna in the artificial lakes of Bohemia, for 

 example, and particularly in the lakes of the Auvergne, as is shown 

 by their geological situation. It is only in this way that we can 

 understand how M. J. de Guerne * could have found in the Azores a 

 perfectly European pelagic fauna in a crater-lake which dates from 

 the fifteenth century. 



Forcl and Pavesi have established two groups which they regard 

 as very distinct in the population of the middle of the lakes — that 

 of the eupelagic species, which live only in the middle, and that of 

 the tychopelagic species, wliich are littoral forms adapted to a life 

 in the open water. According to this division there are in the 

 lakes of Mont Dore only two eupelagic Cladocera, namely Holopedium 

 gibberum and Hyalodaphnia apicata. This second species, however, 

 is very numerous in the littoral zone. This is the cise also with 

 all the Rotatoria enumerated, several of which are regarded as 

 eupelagic by Pavesi. These species, which, according to the defi- 

 nition given, ought only to occur in the middle of the lakes, seem, 

 considering their number, to have adapted themselves to the life of 

 the littoral animals. Ought we to invent for them an analogous 

 term in opposition to the term tycliopelagic ? Probably no one will 

 regard this as necessar)-. 



What is certain is that a great number of species can live equally 

 well in the pelagic region and in the littoral region. In the former 

 case the animals become hyaline, more slender, and better swimmers. 

 Diaptomus castor is a striking example of the well-known fact 

 just referred to. This animal is very abundant on the Mont Dore, 

 and has all the characters of the eupelagic species. It does not 

 occur, according to Zacharias, in the middle of the lakes of North 

 Germany. Sars says that it seems to constitute an exception from 

 the other Calanidse by occurring only in small pools. Lake Pavin, 

 which is 800 metres in diameter, with a depth of 95 metres, should 

 hardly be ranged under this category. It is but small indeed 



* Loc. cit. 

 Ann. & Mag. N. Hist. Ser. 6. Vol. i. 5 



