an, osculant Genus of Crustacea. . 71 



establishment as a subgenus distinct from the Idotca Baffini, I sliall 

 restrict the generic characters to those possessed in common by the 

 two groups. Moreover, as Latreille evidently proposed the genus for 

 the reception of the Arctic species, I shall consider the former as en- 

 titled to a distinct subgeneric name, although, perhaps, in strictness, 

 (in consequence of its being evidently further removed from the 

 typical structure of the adjacent genera than Idotea Baffini,) the ty- 

 pical subgeneric name (adopted from the generic one, as suggested 

 by Mr. Jenyns in the No. of the Mag. Nat. Hist, for March 1834) 

 ought to be given to Oniscus longicornis. 



The follovi'ing are the characters of the genus : 



Corpus elongatum, gracile, cylindricum. 



Antennce 4, internse 2 breves, externse corporis toti saltem longitu- 



dine, divisione ultima articulata, et articulo 4to multo breviori. 

 Pedes antici dissimiles, 2 breves, compressi, supra os applicati. 

 Pedes 6 sequentes mediocres, gracillimi, directione antica, ciliati, 



articulo ultimo praecedentis fere longitudlne, simplici. 

 Pedes 6 postici crassiores, directione postica, unguibus bidentatis. 

 Abdomine segmentis 3, segmentum ultimum elongatum, apice sub- 



ulato. 



The want of mandibular palpi and of vesicular appendages at the 

 base of the legs, as well as the structure of the subabdominal respi- 

 ratory plates covered by a large pair of basal appendages (resembling 

 a pair of cupboard-doors), clearly indicate that this genus belongs to 

 the Isopoda ; whilst the structure and number of segments of the ab- 

 domen and its branchial appendages place it in the family Idoteidce 

 of Leach, in which the only elongate genus is the British Stenosoma, 

 from which the characters above detailed wdll sufficiently remove it. 



The cylindric and elongated form of the body, however, joined to 

 the dissimilar formed legs, the length of the 4th segment of the body, 

 the ovarial pouch beneath this segment in the $ , and the posterior 

 direction of the hind legs, make a much nearer approach to the Lce- 

 modipoda than any other Isopodous genus. 



The genus may be divided into two subgenera. 



to be destitute of legs, and doubts are expressed as to the situation of the genus. 

 The male alone is described. In the New Edinb. Phil. Journ. for October 1827, 

 it is noticed that, as Leacia lacerfosa is identical with Oniscus longicornis, the spe- 

 cific name must be changed ; but that, as the animal was suflSciently generically di- 

 stinguished from the other allied groups, the generic name Leacia might be retained. 

 I believe, also, that Dr. Fleming published a description of the same animal in 1830, 

 under the generic name of Astacella, which must of course be rejected. Amongst 

 the Crustacea preserved in spirits at the British Museum, I have observed specimens 

 incorrectly labelled Corophium longicornc. 



