234 U.S. NATIONAL MUSEUM BULLETIN 271 



difficult character analysis to practice however, and one must use 

 simply a process of elimination by checking generic keys and diagnoses 

 in both families for genera which are not obviously gammarid or 

 eusirid. Five of the genera in couplets 6-18 have reduced inner rami 

 of uropod 3 (melita-like uropod 3) and thus are easily eliminated from 

 consideration as eusirids, for in the latter family, the inner ramus of 

 uropod 3 is always as long as or longer than the outer. 



Key to the Genera of Marine Gammaridae 



Only those genera of Gammaridae with marine representatives are 

 included in this key (except Falklandella) . Some generic pairs, such as 

 Maeropsis-Paraceradocus and Ceradocus-Ceradocoides have not been 

 separated by firm distinctions. Other genera are cited twice in the 

 key because of their intergrading or variable structures. 



One must pursue both pathways in couplet 6 if the species at hand 

 has three accessory flagellar articles. Since other characters are as 

 variable as accessory flagella and more often poorly known, couplet 6 

 has been found to be more satisfactory than conceivable substitutes 

 until some of the poorly described genera are clarified. 



1. Telson entire (fig. 96gf) but often emarginate 2 



Telson cleft (fig. 96h) 6 



2. Urosomal segments with bundles of dorsal spines, metasomites with deep 



serrations on posterodorsal margins Mesogammarus 



Urosomal segments lacking dorsal bundles of spines, metasome lacking 

 serrations but often with carinae 3 



3. Rami of viropod 3 at least 1.5 times as long as peduncle 4 



Rami of uropod 3 equal to or shorter than peduncle 5 



4. Body dorsally carinate (even weakly), article 3 of mandibular palp shorter 



than article 2, lower lip without inner lobes Gammarellus ^ 



Body dorsally smooth, article 3 of mandibular palp longer than article 2, 

 lower lip with inner lobes Weyprechtia 



5. Accessory flagellum 1-articulate, telson broader than long, emarginate, lower 



lip without inner lobes, coxa 4 posteriorly excavate . . . Falklandella 

 Accessory flagellum 8-articulate or more, telson longer than broad, not 

 emarginate, lower lip with strong inner lobes, coxa 4 not excavate pos- 

 teriorly Parapherusa 



6. Accessory flagellum 1- or 2-articulate, occasionally 3 7 



Accessory flagellum 4- or more articulate, occasionally 3 19 



7. Inner ramus of uropod 3 scale-like and much smaller than outer (fig. 96/) . 8 

 Inner ramus of uropod 3 nearly as large as outer 12 



8. Apex of uropod 3 not exceeding extent of uropods 1-2 (fig. 96a) . Melitoides 

 Apex of uropod 3 greatly exceeding extent of uropods 1-2 9 



9. One or more of coxae 1-4 very distinctly longer (almost twice) than coxae 



5-7 Melita 



All coxae subequal in length, very short 10 



1 Gammaracanthus has a telson broader than long but so deeply emarginate as to be considered cleft. 



