112 ISOPODA. 



Amphipoda, which form the subject of our former 

 volume, and the Isopoda, in which the tail is short, with 

 the articulations admitting of little flexion, and with 

 four or five pairs of abdominal appendages, which are 

 lamellar and branchial, and only the posterior styliform. 

 The Isopoda, moreover, are further distinguished by 

 having only the three anterior pairs of thoracic feet di- 

 rected forwards (instead of four, as in the Amphipoda), 

 and four pairs directed backwards. 



These two primary groups he, moreover, considers to 

 be united together by such genera as Tanais, Arcturus, 

 &c., which he affirms agree with the Amphipoda, in 

 having the four anterior pairs of legs directed forwards, 

 and the three posterior pairs backwards ; but, like the 

 Isopods, they have the pleon very short, with only the 

 last pair of members styliform, the others being lamellar 

 and branchial, as in the Isopods. These, therefore, he 

 proposes to form into a third tribe, to be called Aniso- 

 poda. 



As the most recent revision of the animals comprised 

 in the present volume, it will be useful to give the follow- 

 ing condensed sketch of his arrangement of the two tribes, 

 Isopoda and Anisopoda. 

 Tribe 1, Isopoda. 



Subtribe 1, Idotaeidea. 



Fam. 1, Idotaeidae. 



Fam. 2, Chcetilida. 



Subtribe 2, Oniscoidea. 



Fam. 1, Armadillidae. Subfam. 1, Tylin& ; sub- 



fam. 2, Armadillinae. 

 Fam. 2, Oniscidae. Subfam. 1, Oniscinae; sub- 



fam. 2, Scyphacince ; sub f am. 3, Lyginae. 

 Fam. 3, Asellidae. Subfam. 1, Limnorina9 ; sub- 

 fam. 2, Asellinae. 



