AND THE STJKFACE-FILM OP WAT-EK. 5 



and Ostraeoda are quite incapable of opposing the upward pull 

 of the capillary depression. The utmost they can do is to move 

 about horizontally at the surface. 



There are probably still further factors concerned in the 

 floating, such as the amount of convexity of the shell and its 

 degree of water-repulsion ; but these seem only of minor import- 

 ance, and need not be dealt with here. 



Thus far only the danger to which many of the Cladocera and 

 Ostraeoda are exposed when accidentally coming into contact 

 with the surface-film has been considered. Attention must now 

 be directed, however, to some cases in which the peculiar pro- 

 perties of the latter have been so utilised as to play quite a 

 normal part in the economy of the animals concerned. It is 

 impossible to say at present to what extent this utilisation 

 prevails, as very few species have been properly examined from 

 this stand-point. The only genuine cases known to me, where 

 special modifications exist adapting the animals for a life in 

 contact with the surface-film, are to be found in the genera Sca- 

 pholeheris among the Cladocera, and Notodromas (including the 

 Australian NewnTiamia, King) among the Ostraeoda. I have, it 

 is true, seen species belonging to other genera, e. g. Simocej)Jialus 

 vetulus, 0. F. M., and Peracantha truncata, O. P. M., apparently 

 suspended from the surface ; but it seems doubtful if these forms 

 actually make use of the properties of the surface-film ; at any 

 rate, they do not present any evident modifications for that 

 purpose. In the British Isles there is but a single representative 

 of each of the two genera mentioned, namely, ScapJioleberis mu- 

 cronata, 0. P. M. {Daphiia mucronata, Baird), and Notodromas 

 monaclia^ O. P. M. {Gypris ononacha, Baird). These forms, not- 

 withstanding their wide structural differences, have several points 

 in common, correlated, no doubt, with a similar mode of life. 

 But these resemblances will be best appreciated when each 

 species is examined separately in some detail. 



Taking, first, ScapJioleheris mucronata, including both the 

 " acute" and " obtuse rostrata " varieties, it will be seen (PI. I. 

 figs. 1 and 2) that the most conspicuous of its characteristics are 

 the flattened and straight ventral margin, the two long posterior 

 ventral shell-spines, the elevated position of the eye, the remark- 

 able dark coloration of the shell, and the no less remarkable series 

 of modified setae on the ventral portions of the valves. Each of 

 these characteristics has doubtless some significance in connection 



