40 U.S. NATIONAL MUSEUM BULLETIN 2 60 



of the inner ramus of uropod 2 is correlated with a fully elongated 

 coxa 1. The second group of species is assigned to Schisturella 

 {=Pseudonesi7nus). All schisturellas apparently have a similar 

 mandibular molar, coxa 1, uropod 2 and, all except Pseudonesimus 

 abyssi, have a lobate upper lip. The condition of the Schisturella 

 telson varies considerably but this is also true of Zlippoinedon without 

 including the species herein assigned. Lakota adversicola and 

 Schisturella grdbenis resemble Hippomedon in the occurrence of a 

 tooth on pleonal epimeron 3. Arlstiopsis is retained as a distinct 

 genus in its combination of a schisturella-like coxa 1 with hippomedon 

 uroj^od 2 and its nearly chelate gnathopod 1, plus the special con- 

 figuration of its prebuccal outline. 



Lepidepecreopsis Stephensen (1925), a genus fused to ''''Tryphosa''' 

 by Gurjanova (1951), demonstrates the difficulty of accepting the 

 hippomedon and tryphosa sections as monophyletic groups. Lepide- 

 pecreopsis has all the characters of '"''Tryphosa)'' except for the oc- 

 currence of a nearly smooth, asetulose mandibular molar. Lepide- 

 pecreopsis could be assigned to the hippomedon section, but a more 

 prudent course would be to regard it as a subgenus of ^''Tryphosa,'''' 

 Perhaps some of the aberrant species of Hippoinedon have been de- 

 rived from a tryphosa-like ancestry through loss of setules from the 

 mandibular molar. 



A precursor with normally expanded coxa 1 would resemble the 

 type-species of Tmetonyx^ T. cicada (Fabricius). In another direc- 

 tion one may consider that organisms like T. cicada evolved through 

 tapering of coxa 1 into those other species now assigned to Tmetonyx^ 

 which through loss of the accessory tooth of the first gnathopodal 

 dactyl came to resemble species of ''''TryphosaP 



J. L. Barnard (1962a) synonymized Tmetonyx with '"'' Tryphosa''' 

 but a wiser course may be to reestablish Tmetonyx as a monotypic 

 genus situated between Anonyx and '■''Tryphosa''' and to allocate other 

 species of Tmetonyx to a subgenus of ^''TryphosaP The diagnostic 

 characters of these and related genera are summarized in table 2. The 

 evolutionary direction between Tmetonyx cicada and Anonyx is not 

 apparent, as the two genera are so close. Because ridged and cuspi- 

 date mandibular molars are more common in gammarideans than are 

 setulose molars, one must assume that Hippomedon stands closer to 

 a primitive stem than do Tmetonyx^ Anonyx^ and ''''Tryphosa^'' at 

 least as far as that one character is concerned. 



The mandibular molar follows a trend of degeneration in the scheme 

 of table 1. Its strong ridges and strictly cuboidal shape are lost, with 

 the result that the molar becomes slightly flattened, somewhat rounded 

 and bears only a few ridges or superficial imperfections. The condi- 



