ONTSCOIDEA. 713 



Plate 46, fig. 11 a, animal, enlarged; b, part of head, with antennae; 

 c, one of three anterior pairs of legs ; d, fourth pair ; e, fifth pair ; /, 

 outer abdominal plates. 



Rio Negro, Patagonia. Taken from the stomach of a Silurus. 



Length, nine lines. The cephalothorax is oval and broad, and 

 there is an abrupt diminishing of the breadth where the abdomen 

 begins. The last abdominal segment is narrow elongate, with the 

 sides very slightly arcuate, and the apex pointed. The superior 

 antennae, thrown back either side of the body, reach to the second 

 articulation in the thorax, or the third thoracic segment; the first of 

 the slender joints is the longest, and the third the shortest. The setae 

 in the last increase in length towards the apex. The inferior antennae 

 were flexed backward, like the superior ; only four basal joints were 

 observed, and these about equal the flagellum in length. Outer 

 abdominal plates broadest at base ; inner lamella oblong ovate, plu- 

 mosely ciliate. 



The organs of the mouth have a general resemblance to those of 

 the Cleantis. 



Subtribe II. ONISCOIDEA. 



The Oniscoidea are distinguished from the other Isopoda by having 

 the posterior pair of abdominal appendages, which are either styliform 

 or small lamellar, appended as a caudal pair to the extremity of the 

 abdomen. They may be suboperculiform, and cover the ventral surface 

 of the last abdominal segment, as in Tylus ; but they never form a 

 pair of valves covering the other abdominal appendages, as in Idotaea. 



The families included are as follows : — 



Fam. I. Armadillimi. — Corpus bene convexum, stricte articulatum. 

 Abdomen multiarticulatum, segmento ultimo parvo. Appendices 



179 



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