ISOPODS OF NORTH AMERICA, 593 



d. Abdomen not al)ruptly narrower than thorax. E[)iinera of abdominal seg- 

 ments large. 

 e. Body very convex, capable of being rolled up into a i)erfect ball. Articles 

 of flagellum of external antenmesubequal. External branches of the uro- 

 poda ecjual in both sexes. External opercular ranuis of all the abdomi- 

 nal appendages furnished with trache* Genus Cylisticus Schnitzler 



e^. Body more or le.^s dei)ressed, scarcely contractile. Articles of flagellum 

 of external antennjc with the first article generally longer than the 

 second, often subeijual, or even a little shorter. P^xternal opercular 

 ramus of the first and second pairs of abdominal ai)pendages fur- 

 nished with tracheae. 

 /. Second antenna? long; first article of the fiagellum geiu-rally longer 

 than the second. Outer branch of the uropoda flattened, and longer 



in male than in female Genus PorceUlu Latreille 



/■'. Second antenna^ short; flagellum with first article much shorter than 

 second. Outer branch of the uropoda conical, and not longer in 



male than in female Genus LeptotrichiiK Budde-Lund 



cV. Abdomen abruptly narrower than thorax. Epimera of a1)dominal seg- 

 ments small. 

 e. First article of flagellum of external antenna^ generally longer than second. 

 Last abdominal segment reaches sufficiently beyond the epimera of the 

 preceding segment. External opercular ramus of the first and second 

 pairs of abdominal appendages, rarely of the third or of all the pairs, 



furnished with trachea? Genus Metoponorthus Budtle-Lund 



e'. Flagellum of external antenna? with the first article shorter than the 

 second. Last abdominal segment reaches much beyond the epimera 

 of the preceding segment. External opercular ramus of the first and 

 second pairs of abdominal appendages furnished with tracheae. 



Genus Bhi/scotm Budde-Lund 

 c^. Second maxilla more than twice as large as first maxilla and not bilobed 



at the tip Genus IL/perg7iatli>ti<, new genus 



a^. Flagellum of second antennte composed of four articles. 



Genus Actoniscus Harger 



103. Genus ALLONISCUS Dana.^' 



Body rather convex, ver}^ little or scarcely at all contractile. 



Front of head produced in the middle and at the sides into tubercles, 

 the lateral tubercles being prominent and horn-like. Epistome swollen 

 within between the antenna', and slightly carinated. Flag-ellum of 

 second pair of antennte composed of three articles. Frontal marginal 

 line wanting, vertical marginal line passing through the pleurse of the 

 head behind the eyes. 



Epimeral parts of the thoracic segments small, not nuich expanded. 



Abdomen not abruptly narrower than the thorax, with the pleural 

 lamellaj of the third, fourth, and fifth segments large. 



Legs ver}" spinulose. 



«See Budde-Lund for characters of genus, Crust. Isop. Terrestria, 1885, p. 224. 

 28589—05 38 



