Orbicantha Ng & Forges & Lee 2023, n. gen

Genus Orbicantha n. gen. Diagnosis of female. Carapace subcircular; gastrohepatic and frontal regions not elongated, lateral constriction with branchial regions weak (Fig. 19A, B); dorsal surface not swollen, gently convex; mesogastric region with 1 median spine with rounded tip; protogastric region with 2 small spines arranged obliquely; metagastric region with 1 median spine with rounded tip cardiac region with 2 short spines positioned side by side; intestinal region with 1 large median tubercle and 1 short spine lateral to it on each side; hepatic region not swollen with 2 or 3 spines with round tips; branchial region with oblique median row of 3 spines; lateral branchial margin with 6–8 large and many small spines (larger ones round-tipped); dorsal carapace surface with numerous distinct rounded granules (Figs. 19A–E, 22A–C); pterygostomial region protruding anteriorly, forming angular shelf-like structure in lateral view (Fig. 19D, E); suborbital region narrow; pseudorostrum short, as long as supraorbital eave, base wide with 2 distal spines; interantennular spine distinct, tip bifurcated, clearly visible in dorsal view; postorbital spine gently curved with rounded tip; intercalated spine distinct with rounded tip; margin of supraorbital eave with 4 short spines, antorbital spine relatively short, no obvious preorbital spine (Figs. 19B, C, F, 20B); basal antennal article elongate, outer margin lined with short spines, third article short, subquadrate, ca. one-third length of basal article (Fig. 19F); eyes with short, stout peduncle (Fig. 19C, F); epistome is distinctly quadrate. Third maxilliped covered with sharp and rounded granules, those on inner margins sharper, ischium subrectangular, merus subtriangular, proximal margin with strong sharp tubercles, anteroexternal angle expanded, auriculiform (Fig. 20A). Chelipeds slender, relatively short (Fig. 19A, 22A). Ambulatory legs relatively long, slender, P2 and P3 propodus and dactylus with long stiff setae that resemble spines and setae on ventral margins, appearing comb-like (Figs. 19A, 20E, F, 22A), P4 and P5 prominently reduced in size, ventral margins of propodus and dactylus without long spines or long stiff setae (Figs. 19A, 20G, H, 22A). Thoracic sternite 4 with prominent broad V-shaped ridge medially that demarcates sternopleonal cavity (Fig. 14A). Pleon subcircular in shape, domed; covering most of thoracic sternum; somites 1–5 free but barely mobile; somite 6 and telson functionally fused (Fig. 21A, C, D); vulvae on anterior part of sternite 6, subovate, with narrow rim (Fig. 21B). Etymology. The name is derived from the Latin “orbis” for round, alluding to the shape of the carapace; in arbitrary combination with the genus name Pleistacantha. Gender feminine. Remarks. Orbicantha n. gen. is clearly related to the following oregoniid pleistacanthine genera: Pleistacantha Miers, 1879, Parapleisticantha Yokoya 1933, and Pleisticanthoides Yokoya, 1933. It is markedly different from all of these genera in that its carapace shape is semicircular, with the gastrohepatic and frontal regions not elongated (Figs. 19A, B, 22A, B); the pseudorostral spines are very short with the distal spines rounded at the tip (Figs. 19B, F, 22B); the gastric, cardiac and branchial regions possessing distinct spines which have a rounded tip (Figs. 19A–D, 22A–C); the ocular peduncle is very short (Fig. 19F); the third antennal article is very short, being almost quadrate and much shorter than the basal antennal article (Fig. 19F); the epistome is distinctly quadrate (Fig. 19F); the buccal cavity is proportionately much wider (Fig. 19E); and the P4 and P5 are prominently reduced in size and more slender in form compared to P2 and P3, with the propodus and dactylus lacking long spines or setae (Figs. 19A, 20D, E, 22A). In species of Pleistacantha, Parapleisticantha and Pleisticanthoides, the carapace shape is distinctly pyriform, with the gastrohepatic regions separated from the branchial regions by a lateral constriction; the pseudorostral spines usually relatively longer with the distal spines sharp or distinctly tapering; the gastric, cardiac and branchial regions possessing normal sharp spines; the ocular peduncle is elongate; the third antennal article is at least half the length of the basal antennal article, often being subequal in length, being almost quadrate; the epistome is transversely narrower; the buccal cavity is proportionately narrower; and the P5, while reduced in size compared to the other pereopods, is not otherwise different in form, with P3 and P4 similar in form and structure (cf. Guinot & Richer de Forges 1986, 1992; Ahyong & Lee 2006; Ahyong & Ng 2007; Ng & Richer de Forges 2012; Richer de Forges et al. 2013; Ahyong et al. 2015, 2019; Ng et al. 2017). In addition, the ornamentation on most of the dorsal carapace surface of Orbicantha n. gen. consists of small, rounded granules, not spines (Figs. 19B–D, 22B); those on Pleistacantha and Parapleisticantha are invariably covered with small spines. While Pleisticanthoides simplex (Rathbun, 1932) has a spine-covered carapace, the other two species (P. cameroni Ng & Richer de Forges, 2012, and P. piccardorum Ng & Richer de Forges, 2012) have much smoother carapaces with no spines on most of the surface (cf. Ng & Richer de Forges 2012). The presence of four short spines on the supraocular eave (Fig. 19B, C) and two distal spines on the ventral edge of the basal antennal article (Fig. 19F) are characters shared with Parapleistacantha; although they completely differ in the above discussed features (cf. Figs. 15C, 16C, D; Richer de Forges et al. 2013). The differences with the Pleistacantha Miers, 1879 s. lato are obvious, with the latter possessing long pseudorostral spines usually diverging in V bearing long accessory spines versus very short triangular spines; pyriform carapace covered with long sharp spines in Pleistacantha versus rounded carapace with short granule-like spines in Orbicantha n. gen.