Binhthuanomon, Le, Van Tu Do Van Tho & Phan, Doan Dang, 2015

Le, Van Tu Do Van Tho & Phan, Doan Dang, 2015, Binhthuanomon vinhtan, a new genus and new species of semi-terrestrial freshwater crab (Crustacea: Decapoda: Brachyura: Potamidae) from south central Vietnam, Zootaxa 4052 (1), pp. 117-126 : 118

Genus Binhthuanomon , n. gen.

Type species. Binhthuanomon vinhtan , new species, by present designation

Diagnosis. Carapace distinctly transverse, high, dorsal surface strongly convex transversely, longitudinally, surfaces conspicuously smooth, grooves weak to indistinct, regions poorly defined; epigastric, postorbital cristae not separated, indistinct, rounded, smooth; postorbital cristae close to supraorbital margins; postorbital regions narrow; frontal margin strongly deflexed downwards, appears narrow from dorsal view; anterolateral margin rounded, smooth; epibranchial tooth poorly developed, almost absent; external orbital angle low but distinct, triangular. Flagellum of exopod of third maxilliped short, tip blunt, slightly longer than half width of merus. Male abdomen broadly triangular. Male telson with lateral margins concave. G1 sinuous; terminal segment slender, tapered, sinuous without dorsal fold, about 0.26 times length of subterminal segment. G2 with distinct distal segment, shorter than half length of basal segment, basal segment with outer margins slightly convex.

Etymology. The genus is named after the location, the province of Binh Thuan, in combination with the genus name Potamon . The gender is neuter.

Remarks. The general carapace shape and G1 of the new genus are superficially similar to Balssipotamon ungulatum ( Dang & Ho, 2003) , and Villopotamon thaii Dang & Ho, 2003 . It can nevertheless be easily distinguished from these two species by the following characteristics: 1) carapace is high, much inflated and swollen (relatively flat carapace in Balssipotamon and Villopotamon ); 2) H-shaped depression is shallow (Hshaped depression deep in the two genera); 3) epigastric and postorbital cristae are indistinct (epigastric and postorbital cristae distinct, sometimes sharp in the two genera); 4) postorbital cristae is close to supraorbital margins (postorbital cristae not close to supraorbital margins in the two genera); 5) postorbital regions are very narrow (postorbital regions slightly narrow or wide in the two genera); 6) anterolateral margin is rounded and smooth (anterolateral margin sharp and serrated in the two genera); 7) epibranchial tooth is poorly developed, almost absent (epibranchial tooth poorly developed but visible in the two genera); 8) external orbital angle is low (external orbital angle not low in the two genera); 9) groove between sternites 3 and 4 is deep (groove between sternites 3 and 4 shallow in the two genera); 10) lateral margins of telson are concave (lateral margins of telson almost straight in the two genera); and 11) G1 is sinuous (strongly sinuous in Villopotamon or slightly sinuous in Balssipotamon ), with terminal segment slender (terminal segment more slender, sinuous and elongated in Villopotamon or stout and short in Balssipotamon ). Moreover, the new genus lives in semi-terrestrial habitats, whereas Villopotamon and Balssipotamon occupy freshwater habitats.

Dang & Ho (2003) established a new genus, Villopotamon , for a new species, Villopotamon thaii Dang & Ho, 2003 . Yeo & Ng (2007) subsequently included four more species in the genus, all formerly in Potamon sensu lato ( Yeo & Ng, 1999; Dang & Ho, 2003), viz., V. frushstorferi (Balss, 1914) , V. klossianum (Kemp, 1923) , and V. ungulatum ( Dang & Ho, 2003) . Dang & Ho (2008) identified a new genus, Balssipotamon , and compared it with Villopotamon . The new genus Balssipotamon included two species, B. ungulatum ( Dang & Ho, 2003) , and B. fruhstorferi (Balss, 1914) ( Dang & Ho, 2008, 2012). Recently more specimens collected from many localities of southern Vietnam showed a high similarity between these genera in terms of the morphology of the carapace and G1 structures. Further studies are required to clarify the relationship between the two genera.

Binhthuanomon is also superficially similar with Hainanpotamon Dai, 1995 , and Laevimon Y eo & Ng, 2005, owing to its high, swollen, and smooth carapace, and the long, slender ambulatory dactyli. Binhthuanomon can nevertheless be distinguished from Hainanpotamon and Laevimon by the following characteristics: 1) suture between sternites 3 and 4 distinct, broad (not discernible in Hainanpotamon or by a distinct, strong ridge in Laevimon ; and 2) G1 terminal segment filiform, long, without proximal dorsal flap (terminal segment conical in Hainanpotamon or subcylindrical in Laevimon . with proximal dorsal flap).