PROC. BIOL. SOC. WASH. 101(3), 1988, pp. 530-539 ROTUMA LEWISI, NEW GENUS AND SPECIES OF FISH FROM THE SOUTHWEST PACIFIC (GOBIOIDEI, XENISTHMIDAE) Victor G. Springer Abstract.— Rotuma lewisi is described from two specimens, 18.0 and 19.3 mm SL, collected at Rotuma, about 12°30'S, 177°E. Xenisthmid and gobioid synapomorphies are discussed briefly, and some new synapomorphies are hy-pothesized for both groups. The main purpose of this study is to de-scribe a distinctive new genus and species of xenisthmid gobioid fish. The species is known from only two specimens collected during an expedition to the island of Ro-tuma in May, 1986. Springer (1983) defined the gobiid subfamily Xenisthminae (accorded familial status by Hoese 1984), primarily on the ba-sis of three synapomorphies: lower lip with an uninterrupted, free ventral margin, pre-maxillary ascending processes greatly re-duced or absent, and the presence of a rel-atively large rostral bone, which replaces both ascending processes in position and, presumably, function. Homoplasies of these synapomorphies are unknown among other gobioids. Springer (1983) proposed other osteolog-ical characters as possible synapomorphies of the Xenisthmidae, but he was uncertain of them as he lacked (still lacks) skeletal material for two of the four included genera (Allomicrodesmus and an undescribed ge-nus, D. F. Hoese, in litt.): loss of basibran-chials 2-4, hypobranchial 3 lost or vestigial, loss of coronomeckelian and pterosphenoid bones. The new genus and species described below exhibits all these possible synapo-morphies except for two, loss of basibran-chial 4 and the pterosphenoids, which, therefore, should be excluded as synapo-morphies of the family. I now hypothesize an additional xenisth-mid synapormorphy: 5 th ceratobranchial entirely rod-like. All other gobioids I have examined have the teeth-bearing dorsal or posterodorsal surface of the 5th ceratobran-chial considerably broadened. A new go-bioid synapomorphy is hypothesized in the discussion section under character 30. Springer's (1983) Xenisthmidae included Xenisthmus Snyder, Tyson Springer, Allo-microdesmus Schultz, and an undescribed genus and species (D. F. Hoese, in litt.; henceforth, referred to as HUGS) different from the one herein described. Except for Xenisthmus, which comprises about fifteen species, all the xenisthmid genera, are monotypic, and all, with the possible ex-ception of Xenisthmus and HUGS, are rad-ically different from each other in general morphology. Hoese has informed me that HUGS appears to be similar to Xenisthmus, but that HUGS has vomerine and palatine teeth. Because of its striking dissimilarity to the other xenisthmid genera, I have elected to recognize a separate genus for my new species. A cladistic analysis of the Xenisthmidae would be desirable, but is not feasible be-cause of the lack of information on Allomi-crodesmus and HUGS, and because a phy-logenetic classification of higher level gobioid groups (families, subfamilies) has not been hypothesized (i.e., outgroup rela-tionships are unknown). Methods.— yioXhods are those described
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