32 REVIEWS. 



POLYPHEMUS. 



Pour pairs of legs, projecting beyond the carapace, which serves 

 only as a receptacle for the eggs. Head separated from the thorax. 

 The bristles on the large antennas plumose. Post-abdomen produced 

 behind as a long, cylindrical process, with two terminal set*. 



BYTH.OTEEPHES. 



General characters as in Polyphemus, but the anterior legs much 

 longer than the others. The post-abdomen terminating in a single, very 

 long spine. 



PODOtf. 



General characters as in Polyphemus, but the post- abdomen ending 

 in two long spines. 



EVADNE. 



General characters as in Polyphemus, but with the head and thorax 

 united. Post-abdomen very short ; the tail bristles quite small. The 

 receptacle of the eggs very large, and produced into a point. 



Although this classification suppresses several of the unnecessary 

 genera which had been proposed by previous writers, it may be doubted 

 whether the list ought not to undergo a still farther diminution. The 

 number, arrangement, and size of the hairs on the antennas afford, in 

 Entomostraca, excellent specific differences ; but they are surely not of 

 sufficient importance to be used as generic characters. Although I 

 think few naturalists now regard genera as being more than a conve- 

 nient memoria technica, it is evidently desirable that the characters used 

 to separate genera should, throughout the animal kingdom, be as nearly 

 as possible of equal importance, and, to borrow a mathematical expres- 

 sion, of a higher " order" than those by which species are distinguished. 

 Of course this rule can only be applied in a very rough manner, since 

 it is almost impossible to estimate the relative value of different charac- 

 ters ; but it must, I think, be admitted that, if applied, for instance, to 

 the genus Acanthocercus, it would not justify the generic separation of 

 A. rigidus from the species of the preceding genus. 



In the same manner, the groups Macrothrix and Pasithea, so far as 

 the characters given are concerned, are scarcely entitled to rank as sepa- 

 rate genera. 



On the other hand, we must add to the list two genera proposed by 

 Prof. Dana, in his great work on Crustacea. A third, Ceriodaphnia, is 

 scarcely distinct enough from Daphnia. I subjoin the characters given 

 for all three, as many other naturalists may, like Prof. Leydig, have 

 been unable to obtain Prof. Dana's book. 



PENILIA. 



Pedes foliacei numero duodecim. Antennarum posticarum ram} 



