Arcotheres lanensis (Rathbun, 1909), new combination (Figs. 4, 5) Pinnotheres lanensis Rathbun, 1909: 109. — Rathbun, 1910: 332, fig. 14. — Tesch, 1918: 249, 255. — Gordon, 1936: 179. — Suvatti, 1938: 69. — Suvatti, 1950: 159. — Silas & Alagarswami, 1967: 1201, 1223. — Schmitt et al., 1973: 86. — Naiyanetr, 1980: 42. — Naiyanetr, 1998: 104. — Naiyanetr, 2007: 118. — Ng et al., 2008: 250. Type material. Holotype: ZMUC CRU-7022, ovigerous female (cl 2.3 mm, cw 2.6 mm), Koh Lan, 30 fm (= 55 m), mud, coll. Th. Mortensen, 2 March 1900. Description of holotype. Carapace slightly wider than long, soft, in poor condition, irregularly subcircular; front not produced, anterior margin convex in dorsal view; anterolateral margins poorly defined; dorsum smooth, glabrous; dorsal surface convex in profile. Epistome with narrow interantennular septum; median buccal margin with low, obtuse median point. Antennular sinus of similar size to orbit; antennules folded slightly obliquely. Antennal articles 1 and 2 fused to epistome. Eyes not visible in dorsal view, filling orbit, cornea pigmented. Mxp3 ischiomerus length about 1.6× width; outer margin strongly convex; inner proximal margin convex; inner distal margin obtusely rounded. Carpus shorter than propodus. Propodus spatulate, apex rounded, dorsally and distally setose. Dactylus digitiform, distally setose, inserted slightly proximal to propodal midlength, apex not reaching end of propodus. Exopod inner margin almost straight, outer margin convex; flagellum with 2 articles, distally setose. Chelipeds symmetrical from left to right, surfaces glabrous except inner, lower distal margin. Dactylus and pollex relatively straight, crossing distally, without gape; apices simple, not expanded mesially. Dactylus occlusal margin with blunt triangular tooth proximal to midlength, margin straight in distal half, finely denticulate, sparsely setose. Pollex occlusal margin irregularly convex proximally, almost straight in distal half, finely denticulate, sparsely setose; with fringe of short setae on inner ventral margin. Propodus palm dorsal margin 1.5× height, 1.1× length of dactylus; palm ventral margin almost straight. Carpus unarmed. P2–5 unarmed, largely glabrous, setae densest on P5 dactylus. P2, 3, and 5 similar, symmetrical from left to right; right P4 length 1.3× longer than left; relative lengths in decreasing order: P4 (longer) > P5 > P3 > P2. Longer P4 merus about 0.4× pcl. P2–3 dactylus subequal, 0.6× propodus length, longer than half length of P4 and 5 dactyli, evenly arcuate, distally acute, flexor margin unarmed, glabrous. P4 dactylus slender, weakly curved, sparsely setose, unarmed; left dactylus as long as propodus; right dactylus 0.7× propodus length. P5 dactylus slightly longer than propodus, slender, weakly curved, subequal to P4 dactylus; extensor margin with few short setae, flexor margin lined with long setae, unarmed. Relative dactylus lengths: P4 (longer) ~ P5> P3 > P2. Thoracic sternum anterior margin weakly concave medially; sternites 1–3 indistinguishably fused. Pleon of 6 free somites and telson, extending to buccal region, covering bases of P2–P5. Remarks. Pinnotheres lanensis is referred to Arcotheres on the basis of the asymmetrical P4, the elongated and slender P4–5 dactyli that are longer than those of P2 and P3, and the perpendicular articulation between the ischium and merus of P5. Arcotheres lanensis is most similar to A. exiguus, which it resembles in most respects. The two species, however, differ in the proportional length of the P2 and P3 dactyli (distinctly exceeding half the length of the dactyli of P5 and the longer P 4 in A. lanensis, distinctly shorter than half in A. exiguus), as well as the size at maturity. Arcotheres lanensis is already ovigerous by cl 2.3 mm, whereas A. exiguus is not mature until about twice that size (Ng & Ahyong, in preparation). In addition, even in A. exiguus of a similar size to the holotype of A. lanensis, the P5 dactylus is still distinctly more elongated, resembling that of the largest specimens of A. exiguus (Ng & Ahyong, in preparation). Arcotheres lanensis also resembles A. quadratus, A. similis, and A. latus (Bürger, 1895) in the presence of the elongated P2 and P3 dactyli that exceed half the length of the dactyli of P5 and the longer P4, but is readily separated by the unarmed flexor margin of the P5 dactylus (versus one or two rows of spinules on the distal half of the flexor margin in A. quadratus, A. similis, and A. latus). As with A. exiguus, A. similis and A. latus mature at at least least twice the size of A. lanensis, whereas A. quadratus is mature at a similarly small size (Ng & Ahyong, in preparation). Distribution. Presently known only from the Gulf of Thailand.

Published as part of Ahyong, Shane T. & Ng, Peter K. L., 2021, The pinnotherid crabs from the Gulf of Siam described by Rathbun (1909) (Decapoda: Brachyura): revisited and revised, pp. 188-211 in Raffles Bulletin of Zoology 69 on pages 192-195, DOI: 10.26107/RBZ-2021-0016, http://zenodo.org/record/5352100