2 BULLETIN 54, UNITED STATES NATIONAL MUSEUM. 



Synopses are given for the superfamilies, families, genera, and spe- 

 cies. In some cases the synopses do not differ from those which I 

 have previously published for this group. A number of them, how- 

 ever, have been modified. As stated heretofore in previous papers, 

 the synopses of other authors have been used partly or entirely when 

 possible. All the available sources have been investigated and the 

 work of others introduced with few and only necessary changes, 

 in order to adapt it to the material at hand. Schioedte and Meinert's 

 ke} 7 s for the Cymothoidse and the ^Egidse, Hansen's keys for the 

 Cirolanidee and Exocorallanidse, Budde- Lund's keys for the Oniscidse 

 and Armadillididee, Stebbing's keys for the Anthuridae and the Tan- 

 aidas, Benedict's keys for the genus Synidotea and the genus Arcturus 

 have been adapted or followed to a great extent. New species intro- 

 duced into the keys have also caused many modifications. The works 

 of G. O. Bars have been of great service in obtaining definitions of 

 families and genera for use in the synopses and for diagnoses. 



The geographical range included in the present paper is the Atlantic 

 coast of North America to Panama, including Greenland, the West 

 Indies, the Bahamas, and the Bermudas; the Pacific coast of North 

 America to Panama, including Alaska; the fresh-water bodies in North 

 America, and all the territory north of the Isthmus of Panama. 



Most of the illustrations are original, but in many cases the figures 

 of various authors have been reproduced, and due acknowledgments 

 have been recorded in the text in every case. I take pleasure, how- 

 ever, in a collective expression of my thanks and obligations to each 

 and all of the authors whose valuable monographs and memoirs have 

 been utilized in this connection. 



THE ISOPODA. 



The body is dorso-ventrally flattened, and is divided into three parts 

 a head, a thorax composed of seven segments, and an abdomen of six 

 segments. One or two of the segments of the thorax may be united 

 with the head. The head appendages are two pairs of antenna?, a pair 

 of maxillipeds, two pairs of maxilla?, and a pair of mandibles. The 

 maxillipeds are often provided with an epignath on the outside. In 

 the parasitic forms the mouth parts are strongly modified, some of the 

 parts having entirely disappeared. The eyes which are usually present 

 are paired, sessile and compound, sometimes contiguous, sometimes 

 distant. The appendages of the thorax are seven pairs of legs; the 

 last pair are sometimes wanting. The legs are of uniform structure and 

 appearance in the terrestrial forms, but in a large number of forms 

 the first pair, and even the second, third, and fourth pairs differ con- 

 spicuously in structure, length, and function from the succeeding pairs. 

 There are six pairs of abdominal appendages, usually five pairs of 

 pleopods and a pair of .uropods. The pleopoda are in some cases 



