THE TANAIDA 3293 
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a criticism will be withdrawn upon a more careful in- 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-Kdwards, 1829, has eyes, 
several setae 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 perzeon, only three pairs 
of pleopods, and the uropods simple, the single branch 
being two- or three-jointed. 
Leptochelia, Dana, 1849, has two setze on the ‘ palp’ of 
the first maxille, 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. 
Paratanais, 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. 
Pseudotanais, 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 
