234 
CRUSTACEA. 
separated; the superior antennae at least as long as the peduncle of 
the inferior ; the hooks at the end of the feet entire. 
The only species of this subgenus that is known — the Aselle, 
d'eaiL douce, Geoff., Ins. II, xii, 2 ; Squille aselle. Deg., Insect., 
VII, xxi, 1 ; Desmar., Consid., XLIX, 1,2; Idotea aquatica. 
Fab., — is very abundant in fresh and stagnant Avaters as Avell as 
in the marshes, in the vicinity of Paris. Its gait, unless alarmed, 
is very slow. In the spring it issues from the mud in Avhich 
it has passed the winter. The male, much larger than the fe- 
female, carries the latter for eight days, clasping her with the 
fourth pair of feet. When he abandons her she is loaded with 
a great number of ova inclosed in a membranous sac, situated 
under the thorax, which affords an issue to the young through a 
longitudinal fissure. 
Oniscoda, Lat. 
The Oniscodae or Janirae * of Leach differ from the Aselli in the 
approximation of their eyes, in the superior antennae which are 
shorter than the peduncle of the inferior, and in the hooks of the 
tarsi which are bifid. 
The only species known, the Janira macidosa, Leach ; Des- 
mar., Consid., p. 315, was found on the coast of England among 
the Fuel and Ulvae. 
J.ERA, Leach, 
But two tubercles at the extremity of the tail in place of the 
stylets. 
But a single species has been described, the qcera, albifrons, 
Leach; Desm., Consid., p. 316, which is very common on the 
English coast among the Fuci and Ulvae. 
Finally, the Isopoda of the sixth and last section — Oniscides, 
Lat. — have four antennae also, but the two intermediate ones are very 
small, but slightly apparent, and are composed at most of but two 
joints ; the lateral are setaceous. The tail consists of six segments, 
with two or four styliform appendages on the posterior margin of 
the last one, and is without lateral fins. Some of them are aquatic 
and others terrestrial. In the latter, the first leaflets of the under 
part of the tail exliibit a series of small holes, through which air pene- 
trates to the organs of respiration therein contained. 
In some, the sixth joint of their antennae, or the stem, is so com- 
posed, that by counting the little joints of this part the total number 
amounts at least to nine. These Isopoda are marine and form two 
subgenera. The 
Tylos, Lat., 
Appears to possess the faculty of rolling itself into a ball. The 
last segment of the body is semicicvilar, and exactly fills up the 
emargination formed by the preceding one ; the posterior appendages 
* A name employed by Risso for a genus of the same class ; I have consequently 
been obliged to replace it with another. 
