TANAIS. 119 
ISOPODA. TANAIDZ. 
VAGANTIA. 
Genus—TANAIS. (Andouin and Milne Edwards.*) 
Zewxo. TrmpLeton, Trans. Ent. Soc. ii. (1886). 
Crossurus. RatuKe, Nov. Acta. xx. 
Anisocheirus. Wurstwoop, Ann, Sc. Nat. xxvii. (1832). 
Generic character. Body elongated. Cephalon and first seg- 
ment of pereion confluent. Antenne short, subequal. First 
pair of gnathopoda very large, didactyle ; second pair slender, 
simple, and assimilating to the pereiopoda. Pleon five-jointed, 
fourth joint short, fifth terminated by a pair of single-branched 
filamentary uropoda. 
In the large size of the first pair of didactyle legs, 
and in the confluence of the head and first segments 
of the body, this genus agrees with Apseudes, but the 
second pair of legs are slender and simple, and the 
tail is terminated by two short slender appendages com- 
posed of but very few articuli. The antenne are also 
short, being nearly equal in length, although the inner 
or superior pair are much stronger than the outer pair. 
The body is small and has the sides nearly parallel. 
The head and first segment of the body are so closely 
soldered together that little or no trace of their separa- 
tion is visible from above; the anterior portion of the 
head is produced into a point. The eyes are porrected 
on short footstalks, which are movable in the males, 
according to Fritz Miller, but which, according to Van 
Beneden, are completely immovable. The upper or 
inner antenne are seldom so long as the head and fol- 
lowing segment; they are straight, directed forwards and 
downwards, and are much thicker than the lower pair ; 
they are formed of a peduncle which consists of three 
* Résume (not Précis) d. Ent. p. 182 (without description—1829), pl. 
xxix. fig. 1, ‘‘ Tanais de Costa,” not ‘‘Tan. Cavolini.” (The reference by 
M. Edwards to Ann. Sci. Nat. xiii. 288, is erroneous, being intended for 
Rhea.) Latr. Cours d’Ent. 403. 
