PROC. BIOL. SOC. WASH. 107(3), 1994, pp. 445-450 XENISTHMUS BALIUS, A NEW SPECIES OF FISH FROM THE PERSIAN GULF (GOBIOIDEI: XENISTHMIDAE) Anthony C. Gill and John E. Randall Abstract.— Xenisthmus balius is described from nine specimens from Jana Island, Saudi Arabia. It is distinguished from other xenisthmids in having the following characters in combination: dorsal-fin rays VI + 1,13; anterior rim of posterior nostril raised, without well-developed membranous flap; scales of body mainly cycloid; head and body pale, with reticulate, brown mottling dorsally. The Xenisthmidae are a family of small (mostly less than 25 mm SL, and all less than 40 mm SL), sand-diving fishes that live in sand patches adjacent to coral reefs or reef rubble throughout the Indo-Pacific. Several synapomorphies distinguish xen-isthmids from other gobioid fishes and sup-port monophyly of the family: lower lip with uninterrupted, free ventral margin; basi-branchial 2 absent; premaxillary ascending processes greatly reduced; rostral cartilage ossified; and hypobranchial 3 reduced to small cartilage nubbin, or absent (Springer 1983, 1988; Gill (feHoese 1993). The family includes five genera: Allomicrodesmus Schultz, Paraxenisthmus Gill & Hoese, Ro-tuma Springer, Tyson Springer, and Xen-isthmus Snyder (of which Gignimentum Whitley, Luzoneleotris Herre, Platycephal-ops Smith, and Kraemericus Schultz are synonyms). Xenisthmus species are distin-guished from other xenisthmids by a single synapomorphy, third branchiostegal ray with an expanded proximal head, and by the following combination of symplesio-morphies: first dorsal-fin spines 6; scales present on body; and palatine teeth absent (Gill & Hoese 1993). Recent fieldwork by the second author and associates in the Per-sian Gulf has resulted in the discovery of a distinctive new Xenisthmus species, de-scribed herein. Materials and Methods Measurements were made with dial cal-ipers, recorded to the nearest 0.1 mm. All measurements to the snout tip were made to the mid-anterior tip of the snout. Stan-dard length (SL) was measured from the snout tip to the middle of the caudal-fin base. Predorsal, preanal and prepelvic lengths were measured from the snout tip to the base of the anteriormost spine of the relevant fin. Head length was measured from the snout tip to the dorsal edge of the gill opening. Head width is the broadest mea-surement between the posterior edges of the preopercles. Body width was measured at the pectoral-fin base. Snout length was mea-sured over the shortest distance from the snout tip to the orbital rim, without com-pressing the fleshy edge of the latter. Orbit diameter was measured as the horizontal width of the eyeball. Bony interorbital width was the least measurement. Caudal pedun-cle length was measured from the base of the posteriormost anal-fin ray to the ventral edge of the caudal peduncle at the vertical through the posterior edge of the lower hy-pural plate. Caudal peduncle depth was measured obliquely between the bases of the posteriormost anal-and dorsal-fin rays. Pectoral-fin base depth was the vertical depth of the fleshy lobe. Measurements of
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