DESCRIPTIONS OF A NEW GENUS AND SPECIES OF ISOPOD 

 CRUSTACEAN OF THE FAMILY IDOTHEID/E FROM THE 

 MOUTH OF THE RIO DE LA PLATA, ARGENTINA, SOUTH 

 AMERICA. 



By Harriet Richardson, 



Collaborator, Division of Marine Invertebrates, U. S. National Museum. 



In 1888, the U. S. Bureau of Fisheries steamer Albatross, while 

 dredging off the coast of South America, obtained a specimen repre- 

 senting a new genus and species of Idotheidse, the descriptions of 

 which follow. 



CHIRISCUS, new genus. 



Body ovate. Head large, laterally expanded; lateral margins not 

 cleft. First pair of antennae with a peduncle of three articles, the 

 second inserted at the outer lateral margin of the basal article; 

 flagellum composed of a single long article and a minute terminal 

 one. Second antennas concealed by first pair; peduncle composed of 

 five articles and geniculate at the articulation of the second and third 

 articles; flagellum multi-articulate. Maxillipeds with a palp com- 

 posed of three articles. Segments of thorax, except the first, fur- 

 nished with distinct epimera. Seventh segment abruptly narrower 

 than the sixth and not wider than the abdominal segments. Abdo- 

 men composed of three segments, two short ones anterior to a long 

 terminal segment. First pair of legs strongly prehensile, with pro- 

 podus large and dilated. Four following pairs and seventh pair 

 similar, with terminal joints furnished with long hairs; these legs 

 have no dactylus. Sixth pair of legs much longer than the others, 

 with the carpus and propodus elongate. 



This genus is similar to both MacrocMridotea Ohlin 1 and to 

 Chsetilia Dana. 2 It differs from both, however, in not having the 

 second and third pairs of legs prehensile, and in having no dactylus 



Jlsopoda from Tierra del Fuego and Patagonia. Svenska Expeditionen till Magellansliinderna, vol. 

 2, No. 11, 1901, pp. 286-291. 

 "IT. S Expl Exp, vol. 14, 1853, pp. 711-713, pi. 46, fig. 11a-/. 



Proceedings U. S. National Museum, Vol. 40— No. 1811. 



169 



