Type species. Bythiopagurus macrocolus sp. nov.

Diagnosis. Shield and carapace lateral lobes well calcified; posterior carapace with posterior carapace element contiguous or fused with lateral carapace lobes, posteromedian and posterolateral plates also well calcified at least in anterior halves (Fig. 1a). Sulci cardiobranchialis extending approximately 0.50 length of posterior carapace; cardiac sulci extending nearly to posterior margin. Cervical groove distinct; linea transversalis usually partially discernable externally; linea anomurica not readily apparent in posterior portion of carapace. Frequently small areas of branchiostegal regions weakly calcified.

13 pairs of distally quadriserial (Fig. 1b) phyllobranchiate gills.

Ocular peduncles short and stout; ocular acicles acutely triangular. Antennal peduncles with supernumerary segmentation.

Mandibles sometimes weakly asymmetrical (Fig. 1c, d); strongly calcified; palp well developed. Maxillule (Fig. 1e) with anterior lobe of endopod weakly developed, posterior lobe moderately well developed, not recurved. Maxilla (Fig. 1f) with slender scaphognathite. First maxilliped (Fig. 1g) with slender two-segmented exopod; no epipod. Third maxilliped with basis and ischium not fused; crista dentata of ischium well developed (Fig. 1h), 1 or 2 accessory teeth.

Chelipeds subequal; left sometimes longer, but not stronger; dactyls and fixed fingers opening in horizontal plane. Fourth pereopod very weakly semichelate; propodal rasp consisting of single row of corneous scales (Fig. 2c). Fifth pereopod chelate. Male usually with papilla or very short sexual tube extruded from both right and left gonopores (Fig. 2d). Female with paired gonopores.

Abdomen somewhat reduced, dextrally twisted. Male with left unpaired pleopods on somites 3–5; exopods moderately long and very slender, endopods reduced. Female with paired and modified pleopod 1, unpaired left pleopods 2–4 with both rami of approximately equal length, endopods much thicker; pleopod 5 as in male.

Uropods markedly asymmetrical. Telson with transverse indentations; posterior lobes separated by median cleft; terminal margins each with few small spines.

Etymology. From Greek bythios meaning from the deep, and pagurous meaning crab, and referring to the archibenthic and archiabyssal depths from which the type species was collected.