MARINE GAMMARIDEAN AMPHIPODA 45 



trated in figures 1 and 2 and diagnosed on p. 50. Almost all other 

 families are defined by combinations of characters expressing either 

 minor specializations or minor simplifications of the basic gammari- 

 dean plan. 



The diagrammatic key to families. — This key (figs. 1-53) is 

 composed of 54 boxes, 51 of which represent a family or superfamily 

 containing one or more illustrated characters in solid lines that dis- 

 tinguish it from the basic gammaridean (figs. l-2b). For example, if 

 mouthparts are not illustrated they either resemble those of the 

 basic gammaridean or are so highly variable that they have no diag- 

 nostic differences from the basic gammaridean and are thus not 

 mutually exclusive. Each of these concordant families is noted in the 

 captions of the illustrated key and character differences in broken 

 lines are figured and explained where necessary. The diagrammatic 

 key is, therefore, not an absolute endpoint, for the taxonomist must 

 also check the illustrated boxes of families resembling the first pro- 

 visional identification. 



The diagrammatic key is arranged in a way to deal first with those 

 families which have highly characteristic and easily observed morphol- 

 ogy. Thus, Lysianassidae (fig. 3), with their characteristic gnathopod 

 2 are presented first, followed by the similar Stegocephalidae (fig. 4), 

 which have characteristic coxae, head, mouthparts and mandibles, 

 followed by Ochlesidae (fig. 5) without maxillipedal palp, Lafystiidae 

 (fig. 6) with 2-articulate maxillipedal palp, and Acanthonotozomatidae 

 (fig. 7) with characteristic coxae. Following those groups is a pair of 

 families with cylindrical bodies, succeeded by seven families with 

 uniramous uropod 3. Succeeding that point (Amphilochidae, fig. 19) 

 all families have a biramous uropod 3, rarely with exceptional genera 

 or species. Other characters are abnormal to the basic gammaridean, 

 however, such as the small coxa 1 of Amphilochidae and some Leuco- 

 thoidae and the characteristic gnathopod 1 of Leucothoidae and 

 Anamixidae. There follows a group of families with various combina- 

 tions of coalesced urosomites, and degraded mandibles or acuminate 

 coxae. Commencing with the Oedicerotidae (fig. 27) the identifications 

 become increasingly difficult as the differentiations from the basic 

 gammaridean become less apparent : the Oedicerotidae have elongated 

 pereopod 5 and short, uncleft telson; Paradaliscidae (fig. 28) have 

 characteristic mandibles; Bateidae (fig. 30) have degraded gnathopod 

 1; Synopiidae (fig. 31) have massive, rostrate heads; the next four 

 families (figs. 32-35) have various combinations of feeble gnathopods, 

 degraded mandibles, special shapes of lower lips and maxillae and 

 various telsonic lengths and clefts; the next three families differ from 

 the basic gammaridean by poorly cleft telsons and degraded accessory 

 flagella; the next group has cleft telesons but degraded accessory 



