ISOPOD8 OF NORTH AMERICA. 593 



d. Abdomen not abruptly narrower than thorax. Epimera of abdominal seg- 

 ments large. 



e. Body very convex, capable of being rolled up into a perfect ball. Articles 

 of nagellum of external antennae subequal. External branchesof the uro- 

 poda equal in both sexes. External opercular ramus of all the abdomi- 

 nal appendages furnished with tracheae Genus Cylisticus Schnitzler 



e f . Body more or less depressed, scarcely contractile. Articles of nagellum 

 of external antennae with the first article generally longer than the 

 second, often subequal, or even a little shorter. External opercular 

 ramus of the first and second pairs of abdominal appendages fur- 

 nished with tracheae. 



. /. Second antennae long; first article of the flagellum generally longer 

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



in male than in female Genus Porcelllo Latreille 



/'. Second antennte short; flagellum with first article much shorter than 

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



male than in female Genus Leptotrichus Budde-Lund 



d f . Abdomen abruptly narrower than thorax. Epimera of abdominal seg- 

 ments small. 



e. First article of flagellum of external antennae generally longer than second. 

 Las't 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 tracheae ....'.... Genus Metoponorthus Budde-Lund 



e' '. Flagellum of external antennae 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 Rhyscotus Budde-Lund 

 c 7 . Second maxilla more than twice as large as first maxilla and not bilobed 



at the tip Genus Hypergnathus, new genus 



a' '. Flagellum of second antennae composed of four articles. 



Genus Actomiscus Harger 



1O3. Genus ALLONISCUS Dana." 



Body rather convex, very 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 antennae, and slightly carinated. Flagellum of 

 second pair of antennae composed of three articles. Frontal marginal 

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

 head behind the eyes. 



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



Abdomen not abruptly narrower than the thorax, with the pleural 

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



Legs very spinulose. 



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

 2858905 38 



