592 BULLETIN 54, UNITED STATES NATIONAL MUSEUM. 



Family XXV. 0N1SC1D^E.« 



Body oval or oblong, rather convex, very little or scarcely contract- 

 ile, very rarely convex; with difficult}" contractile into a ball. 



Second pair of antennae generally long, with the Hagellum composed 

 of only a few articles. Antennal foramina large. Pleural parts of the 

 head distinctly separated by a vertical marginal line and an infra- 

 marginal line. Cypeus arched. Eyes generally well developed, com- 

 pound. Molar expansion of mandibles obsolete, without any triturat- 

 ing surface, it being replaced by brush-like sette. First maxilla with 

 the outer lobe tipped with spines, the inner lobe furnished with two 

 plumose processes. Second maxilla? bilobate at the tip. Maxillipeds 

 with palp composed of three articles and but little longer than masti- 

 catory lobe; masticatory lobe truncate at tip; epignath large. 



There are six thoracic segments in the .young before they are 

 hatched. 



The legs are generalh" long. The uropoda are produced, always 

 extending beyond the terminal segment and the preceding segment. 



First and second pairs of pleopoda moditied in the male, the inner 

 branch of the second pair terminating in a slender stylet. 



ANALYTICAL KEY TO THE GENERA OP THE FAMILY ONISCID^. ^ 



a. Flagellum of second antenna^ composed of less than four articles. 



b. External opercular ramus of the a])dominal appendages containing no special 

 respiratory organ. Flagellum of external anteniu* triarticulate. 

 c. Kpimera of thoracic segments large, with all the posterior angles acute. 

 Abdomen not abruptly narrower than thorax. First two abdominal seg- 

 ments very short, three following ones large, with large acute epimera. 

 d. Front of head produced at the middle and at the sides in tubercles; lateral 



tubercles horn-like Genus AUoniscus Dana 



(V. Front and sides of head not produced in tubercles. With or without lat- 

 eral lobes. 



€. Surface of body smooth Genus L;/probius Budde-Luml 



e^. Surface of body granulated or tuberculate. 

 /. Terminal segment of body short, widely rounded posteriorly. Basal 

 article of uropoda broadly expanded inside. Inner branches contig- 

 uous along their inner lateral margins.. Genus Synuropus Richardson 

 f\ Terminal segment of body conically produced. Basal joint of uro- 

 poda oy)long Genus Oniscus Linnaeus 



c^. Epimera of thoracic segments small. Abdomen abruptly narrower than 

 thorax; first two segments generally equal in length to those following; 



epimera very small, but manifest Genus J'lnlosda Latreille 



//. External opercular ramus of the first and second pairs of abdominal append- 

 ages furnished with trachete. Flagellum of external antenn;e biarticulate. 

 c. Second maxilla not larger than first maxilla, and bilobed at the tip. 



«See Budde-Lund for characters of family, Crustacea Isopoda Terrestria, 1885, p. 

 75, and G. O. Sars, Crustacea of Norway, II, 1899, pp. 169-170. 



^The genus Acanthoniscus (White) Kinihan is not included in the key. 



