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'eau 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 waters as well 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 which 

 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 Janirse * 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 maculosa, Leach ; Des- 

 mar., Consid., p. 315, was found on the coast of England among 

 the Fuci and Ulvae. 



Leach, 



But two tubercles at the extremity of the tail in place of the 

 stylets. 



But a single species has been described, the gezra albifrons, 

 Leach; Desm., Consid., p. 316, which is very common on the 

 English coast among the Fuci and Ulvse. 



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 exhibit 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 semicicular, 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. 



