592 BULLETIN 54, UNITED STATES NATIONAL MUSEUM. 



Family XXV. ONISCID^E." 



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

 ile, very rarely convex; with difficulty contractile into a ball. 



Second pair of antennae generally long, with the flagellum composed 

 of only a few articles. Antennal foramina large. Pleura! 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 setae. First maxillae with 

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

 plumose processes. Second maxillae 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 generally long. The uropoda are produced, always 

 extending beyond the terminal segment and the preceding segment. 



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

 branch of the second pair terminating in a slender stylet. 



ANALYTICAL KEY TO THE GENERA OF THE FAMILY ONI8CID.B. & 



a. Flagellum of second antennae composed of less than four articles. 

 b. External opercular ramus of the abdominal appendages containing no special 



respiratory organ. Flagellum of external antennae triarticulate. 

 c. Epimera 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 Alloniscus Dana 



d'. Front and sides of head not produced in tubercles. With or without lat- 

 eral lobes. 



e. Surface of body smooth Genus Lyprobius Budde-Lund 



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 

 /'. Terminal segment of body conically produced. Basal joint of uro- 

 poda oblong Genus Onwcus Linnaeus 



</. 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 Philosda Latreille 



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

 ages furnished with tracheae. Flagellum of external antennae 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. 



