( 15 ) 



2nd. The mode of articulation of the Bo-called external appendage 



(ischium), and the peduncle (basis). 



This is always at the extremity of the peduncle, and generally on a 

 plane inferior to that of the so-called internal appendage, the peduncle 

 being hollowed out into a regular joint for its reception, and the exter- 

 nal border of the peduncle terminating inferiorly in a tooth outside the 

 articulation ; the internal, on the other hand, arising from a lateral pro- 

 cess, more or less distinctly marked, and generally on a plane much 

 superior to the articulation of the external appendage. This is evident, 

 even in such forms as Ligia, where the apex of the peduncle being trun- 

 cated, the appendages arise from nearly the same point, the lateral pro- 

 cess being slightly larger than the real apex, or as Ligidium, in which the 

 lateral process is still further elongated, so that the ischium appears 

 to arise above the internal appendage. 



From this extreme we trace the lateral process till we arrive at such 

 forms as Oniscus, where, without careful examination, we might doubt 

 its existence at all, and where, to a superficial examination, the internal 

 appendage appears to have no connexion with the peduncle at all, but 

 rather to spring directly from the last ring : but more of this genus anon, 



3. The difference in form and relations between the external 

 (ischium) and internal (accessory filament) appendages. 



Besides the difference between these two, displayed in the fact of the 

 one (ischium) being articulated to the extremity j the other (accessory 

 filament) to the lateral process of the peduncle, the differences of the two 

 are remarkable. 



The external (ischium) is generally more or less compressed, often 

 abrupt at its termination, and generally ending in a tuft of hairs. 



The internal (accessory filament), on the other hand, is rounded, 

 spine-like, generally made up of more than one articulation, and termi- 

 nates in a hollow hair, or rather becomes filamentous, pressenting in 

 short, all the characters of a true accessory appendage ; it is from this 

 character I have named it the accessory filament. 



Having established these relations, next for a detailed description of 

 the entire organs, i. e. first, the peduncle or basis ; second, ischium ; 

 third, accessory filament. 



1 . The Peduncle ar BaMs. — This is attached to last abdominal ring, 

 and is either somewhat (quadrilateral, without distinct lateral process, 

 its inner margin dilated into a lobe (Ligia). 



Or else somewhat triangular ; its intenial margin produced into a 

 rounded process, which bears the accessory filament (Philoscia, Oniscus). 



The lateral process (obsolete in Tylos) is either distinct, produced 

 beyond, and arising from inner angle of the apex of the peduncle (Ligi- 

 dium), arising from near the base of the peduncle (Oniscus, Porcellio); 

 or else indistinct, and almost on the same level as the apex (Ligia); be- 

 tween these, every gradtition of development exists. The peduncle may, 

 in fine, be described in general terms as a flattened articulation, gene- 

 rdly furnished internally with an accessory appendage, bearing the 

 ischium on its summit, and more or less coyered by the last abdominal 

 ring, from whence it arises. 



