TIIE TANAIDiE 323 



Within recent years the genera of this family have 

 become tolerably numerous. Many of the species are 

 quite microscopic, and in some instances an impatient 

 observer may be tempted to suppose that the distinguish- 

 ing features even of the genera have been invented to 

 match the size of the specimens. But in most cases such 

 a criticism will be withdrawn upon a more careful in- 

 spection. 



Only a few selected characters can here be offered to 

 discriminate the fifteen (or rather, the fourteen) genera 

 that have been established : — 



Tanais, Audouin and Milne-Edwards, 1829, has eyes, 

 several setas on the ' palp ' of the first maxilla, the marsu- 

 pium of the female consisting of two large plates affixed 

 to the fourth free segment of the perajon, only three pairs 

 of pleopods, and the uropods simple, the single branch 

 being two- or three-jointed. 



Leftochelia, Dana, 1849, has two setae on the ' palp ' of 

 the first maxillae, the first gnathopods of the adult male 

 with very elongate thumb and finger, the marsupium of 

 the female consisting of a pair of plates on each of the 

 first four free segments, five pairs of pleopods, and in the 

 uropods a multiarticulate inner and a one-jointed outer 

 branch. 



Heterotanais, Sars, 1880, scarcely differs from Lepto- 

 chelia, except in the first gnathopods of the male, which 

 are incompletely chelate, the so-called thumb being very 

 little instead of enormously produced. In the uropods the 

 outer branch is little but two-jointed, the inner filiform, 

 with four or five joints. 



Pwratanais, Dana, 1852, has eyes, but without distinct 

 ocular lobes. The mandibles are strong in the female, 

 with cylindrical molar process, whereas in the male all the 

 mouth-organs, except the maxillipeds, are lost. The first 

 gnathopods are similar in the two sexes. Of the uropods 

 both branches are two-jointed. 



Psevdotcmais, Sars, 1880. The eyes are imperfect or 

 absent, and there are no ocular lobes. The mandibles 

 have a stiliform molar. The maxillipeds are fused at the 



