44 U.S. MATIONAL MUSEUM BULLETIN 271 



The question of interest is whether the Tahtroidea existed simul- 

 taneous with these events or whether they presumably came much 

 later, and indeed whether they evolved directly through the gammarid 

 funnel; whether modern Gammaridae truly represent a base stock 

 to all other Gammaridea or whether they are simply a blind-alley 

 sidebranch coincidentally representing today the logical primitive 

 gammaridean. 



That Gammaridae and all other Amphipoda are heavily oriented 

 to cold water seems more than happenstance. We may determine 

 eventually that the tropics have the most numerous species of Am- 

 phipoda of any province but there is little current evidence that we 

 shall also find tropical genera to be the most numerous. Cold waters 

 (but not high polar) undoubtedly have the greater diversity at the 

 higher taxonomic levels. It is occasionally, or at least not always, 

 true that biotic groups have their highest diversity in their centers 

 of origin and I do not wish to imply that diversity and centers of 

 origin are correlated in the Amphipoda. Amphipoda have a bimodal 

 diversity anyway as cold waters obviously occur in two circum- 

 ferential bands. But the facts that the most primitive morphotype 

 has its center of diversity in cold Siberian Seas, that Gammaridae 

 have strongly radiated generically only in a cold Siberian lake and 

 not in isolated tropical lakes, that Nearctic-Palearctic fresh-water 

 Amphipoda clearly are more diverse generically than in tropical 

 fresh waters, that marine gammarid genera are most numerous in 

 cool waters, that a whole suborder of amphipods, the Hyperiidea is 

 confined primarily to cool pelagic waters, that the largest family of 

 marine amphipods, the Lysianassidae is confined largely to cool 

 waters, all suggest that Amphipoda are preadapted to cool waters 

 and have undergone the major "post-amphipodan" radiation in such 

 climes. 



Identification Procedures 



The identification of a gammaridean amphipod even at familial 

 levels so often requires a complete dissection and analysis of all 

 appendages and mouthparts that the procedure is considered to be 

 mandatory. This handbook cannot be utilized successfully by a non- 

 specialist without dissecting appendages (Appendix I) and observing 

 minute characters (checklist of Appendix II). Once the student has 

 gained some experience, however, portions of these procedures may 

 be skipped, for some families and many genera can often be recog- 

 nized without extensive analysis. 



The procedures for identification described herein are manifold. 

 They include the memorization of a basic gammaridean plan, illus- 



