Revision of the sesarmid crab genera Labuanium Serène and Soh, 1970, Scandarma Schubart, Liu and Cuesta, 2003 and Namlacium Serène and Soh, 1970 (Crustacea: Decapoda: Brachyura: Sesarmidae), with descriptions of four new genera and two new species Author Naruse, Tohru Author Ng, Peter K. L. text Journal of Natural History 2020 J. Nat. Hist. 2020-08-12 54 7 - 8 445 532 http://dx.doi.org/10.1080/00222933.2020.1763491 journal article 55729 10.1080/00222933.2020.1763491 fb05d004-0f2d-44da-b91e-b6e8304ab2a5 1464-5262 4609148 414B8DAA-584F-4070-A355-83B583D0D017 Genus Namlacium Serène and Soh, 1970 Namlacium Serène and Soh, 1970 , p. 404 ; Manning and Holthuis 1981 , p. 242 ; Ng et al. 2008a , p. 221 ; Ng et al. 2017, p. 102. Type species Sesarma crepidatum Calman, 1925 , by original designation. Gender neuter. Included species Namlacium crepidatum ( Calman, 1925 ) . Diagnosis Carapace subcircular, wider than long; lateral margins convex, external orbital angle very sharp, long, weakly upcurved distally, directed anteriorly, no trace of epibranchial tooth; dorsal surface weakly convex longitudinally and transversely, glabrous, regions moderately defined. Front narrow, about one-third of frontorbital width, deflexed at anterior margin of postfrontal lobes, distally recurved, directed anteriorly; frontal margin entire, straight in dorsal view, overhanging onto antennular septum and fossae. One pair of postfrontal lobes present, these lobes far from frontal margin; lateral part of front with longitudinal low cristae consisting of row of 3 small granules. Antennular septum narrow, short. Orbit, in dorsal view, deep J-shaped, median part of supraorbital margin strongly oblique; inner orbital tooth short, directed dorsoanteriorly. No longitudinal ridge on ventral surface of external orbital angle. Epistome posterior margin with 3 triangular lobes, lateral lobes directed anteroventrally, median lobe directed ventrally. Antenna entering orbit through wide gap between inner orbital tooth and front. Mxp3 exopod with distinct flagellum. Chela palm of male with short to long, interrupted longitudinal rows of weak granules on upper surface extending from hinge with carpus to inner base of dactylus; granules scattered on upper to outer surfaces larger than those forming longitudinal rows; outer surface generally convex, granulated, except for slightly depressed area around base of fingers, low oblique protuberance proximal to this depressed area; inner surface with C-shaped row of large tubercles extending from distal upper corner to median part of palm; thick rim extending along occlusal margin of immovable finger to dactylar articulation on both outer and inner surface, thick rims of both outer and inner surfaces not interrupted near dactylar articulation. Upper-inner margin of movable finger with sparse, low granules over proximal half. Ambulatory legs (P2–5) long, dorso-ventrally flattened, slender; distal anterior corner of meri forming distinct triangular projection, both anterior and posterior margins bearing sharp subdistal tooth; carpi and propodi not distinctly narrower than respective meri; dactyli very short, shorter than about quarter length of respective propodi, proximally bent inward almost at 90°, extensor surface covered with dense mat of brush-like, stiff setae. Male thoracic sternites transversely wide; sternite 8 clearly exposed. Male pleon wide, lateral margins of somite 3 to proximal two-thirds of somite 6 straight; telson reaching proximal half of bases of cheliped coxae. G1 short, stout, slightly constricted medially, slightly curved outward distally, distally with corneous, rectangular beak-like process, this process directed distolaterally. Remarks When Calman (1925) described Sesarma crepidatum , he noted that the generic assignment of this species to Sesarma was provisional. Serène and Soh (1970) agreed and referred it to a monotypic new genus, Namlacium , commenting that the genus was close to Labuanium in many key characters and that they may eventually be found synonymous. However, the species is one of the most distinctive in Sesarmidae because of the sharply pointed external orbital angle, inflated lateral margins of the carapace, and markedly short, proximally bent dactylus of the ambulatory legs with dense mat of brushlike setae on extensor surface. Although Serène and Soh (1970) believed that Namlacium was closely related to Labuanium , the two genera are actually quite distinct from each other in a number of characters in addition to the above-mentioned characters – that is, the outline of the carapace (subcircular with convex lateral margins, and wider than long in Namlacium , Figure 35 (a); vs elongated, rectangular, longer than wide in Labuanium , Figure 2 (a)), the structure of the front and antennular septum (narrow antennular septum overhung by the frontal margin in Namlacium , Figure 35 (a); vs wide antennular septum exposed and attached to the median concavity of frontal margin in Labuanium , Figure 2 (b)), the structure of the postfrontal lobes (divided into two lobes, far from frontal margin in Namlacium , Figure 35 (a); vs divided into four lobes, projected anteriorly and overhanging frontal margin in Labuanium , Figure 2 (a)), ambulatory legs (more slender ambulatory legs with dactyli being bent proximally at 90° in Namlacium , Figure 36 (d, e); vs wider ambulatory legs with straight dactyli in Labuanium , Figure 2 (a)) and male pleon (wide in Namlacium , Figure 37 (a); vs narrow in Labuanium , Figure 4 (a)).