MARDSTE GAMMARIDEAN AMPHIPODA 35 



molar grinding ridges, miniaturization of coxae, and simplicity of 

 gnathopods to their definitive extremes. Bateidae have the cephalic 

 pleustid aspect but have undergone strong divergence through reduc- 

 tion of gnathopod 1 to a single article or two. 



Paramphithoidae have links to more advanced families, either 

 directly or indirectly. Generic intergradations between Paramphi- 

 thoidae and Astyridae are readily apparent (see those families for 

 elaboration). The Lepechinellidae have the acuminate coxae of par- 

 amphithoids, but through coalescence of two urosomal segments 

 resemble a parallel grade, the Atylidae. Their origin is thus plotted in 

 graph 1 as a mixture of characters between Atylidae and Paramphi- 

 thoidae, an unsatisfactory procedure indicating not phyletic affinities 

 but gradational structure. 



The body plan of the Stilipedidae conforms remarkably to that of 

 the Astyridae. If the pardaliscid left mandible, adopted by the 

 Stilipedidae, is a character of major evolutionary significance, then 

 one might consider that stilipedids stand close to the Pardaliscidae. 

 But the astyrid mandible is in a condition procursive to the pardaliscid 

 right mandible. It is partially flattened and grossly toothed while 

 retaining a nontriturative molar. It thus resembles the pardaliscid 

 right mandible whereas the stilipedid mandibles resemble the broad- 

 ened untoothed left mandible of pardaliscids. Since pardaliscids combine 

 both kinds in a single individual, it does not stretch one's imagination 

 to consider that the astyrid mandible evolved into that of stilipedids. 

 That astyrids did not also produce pardaliscids is attested to by the 

 strong accessory flagellum and weak coxae of pardaliscids. 



Atylidae have the same general advancements of the eusirids but 

 the fusion of two urosomites marks a line of evolution which some 

 students suggest has led to the inquilinous Dexaminidae lacking 

 mandibular palps; this condition naturally leads to the palpless 

 Prophliantidae in which aU urosomites have become coalesced. 



A more radical inquUinous specialization in the acanthonotozomatid 

 line seems to have its gravitational balance near the Paramphithoidae. 

 Here a marked change in the mouthpart field from its basic quadrati- 

 form bundle to a conical or triangular field suggests increasing stages 

 towards the functions of piercing and sucking. The individual mouth- 

 parts increasingly become better adapted for those habits in the various 

 acanthonotozomatid genera through incisorial styliformity and reduc- 

 tion of molars; but most acanthonotozomatids live in south polar 

 waters, often reach giant body proportions, and, in many cases, the 

 mandibles have become broad, flattened, and very powerful as if their 

 bearers have evolved toward a state of subpredation, perhaps on giant 

 sessile organisms such as sponges. They might be called predatorial 

 grazers if this habit is confirmed. 



