FKESH WATER CYCLOPID.E AND CALANIDiE. 45 



good specific marks. First, as regards the frontal filaments, 

 which are relied upon as furnishing an important character, and 

 even a name, to D. bifilosus. These organs are apparently 

 merely the segments of a bifid, very finely divided rostrum : 

 they are so extremely slender as to be (in spirit specimens, at 

 any rate) often very difficult to see, and though in some few of 

 the Burgh Marsh specimens I have succeeded in finding them, 

 in most cases I quite failed to do so, and believe that they are 

 usually absent, though in other respects the specimens from that 

 locality entirely agree with D. bifilosus. Claus and Boeck, 

 however, notice the occurrence of frontal filaments in "D. longi- 

 remis, Lilljeborg." Secondly, The spines of the last thoracic 

 segment of (the restricted) longiremis are rarely (never, in my 

 experience) developed so largely as represented by Giesbrecht ; 

 usually they are so small as to be very easily overlooked. 

 Thirdly, The proportions of the furca are certainly very variable, 

 and even in extreme forms (except in discaudatus) are not well 

 enough marked to be of much diagnostic service. Fourthly, 

 The diversity of size in the fifth feet of the two sexes cannot be 

 looked upon as of much moment, and the differences of form of 

 those organs in the three species are certainly by no means well 

 marked. I therefore think that the two forms bifilosus and dis- 

 caudatus, though presenting characters of very great interest, 

 should be looked upon as races or varieties of the original type 

 and not as separate species. And a nomenclature which retains 

 such forms as varieties in direct connection with a central type, 

 so preserving the idea of relationship and evolution, is not only 

 truer to the actual facts, but adds a distinct and vivifying in- 

 terest to the mere dry bones of classification. 



I have no note of the occurrence of any species of Acartia in 

 fresh or brackish water except in one locality, Burgh Marsh, 

 Cumberland, where I took the bifilosus form abundantly many 

 years ago. Eurytemora affinis occurred in the same pools and 

 in equal abundance. 



The generic name Acartia was proposed by Dana in 1846, and 

 was undoubtedly meant to cover the forms more lately assigned 

 by Prof. Lilljeborg to the genus Bias. Having the claim of 



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