Author: Smith and Radcliffe, 1912
Diagnostic Features:
Essentially those of Apristurus, but without a first dorsal fin (possibly abnormal, see below). Body not tadpole-shaped, stocky and compressed, increasing in height up to the pectoral and trunk region and tapering posteriorly; body soft and flabby, with thin skin and weakly calcified dermal denticles; stomach probably not inflatable; tail short, length from vent to lower caudal origin about 3/5 of snout-vent length. Head greatly depressed, pointed and wedge-shaped in lateral view; head elongated, but slightly less than 1/4 of total length (adult male); snout elongated, its length greater than mouth width, greatly flattened, narrow and pointed in lateral view; snout expanded laterally, narrowly spatulate and bell-shaped in dorsoventral view; ampullal pores enlarged and prominent on snout; nostrils enlarged, with incurrent and excurrent apertures broadly open to exterior; anterior nasal flaps reduced to angular lobes, without barbels, widely separate from each other and falling far anterior to mouth; internarial space 0.9 times in nostril width; no nasoral grooves; eyes dorsolateral on head, broad subocular ridges present below eyes; mouth broadly arched, with lower symphysis well behind upper so that upper teeth are exposed in ventral view; labial furrows present along both upper and lower jaws; these long and reaching to level of upper symphysis of mouth; branchial region not greatly enlarged, distance from spiracles to fifth gill slits less than half head length; gill slits lateral on head. Origin of second dorsal fin about over the anal midbase; pectoral fin width less than mouth width; inner margins of pelvic fins not fused over claspers in adult males; claspers short, thick, and distally pointed, not extending more than 2/ 3 of their length behind the pelvic fin tips; anal fin enlarged and greatly elongated, larger than pelvic fins and dorsal fin, base length over twice second dorsal base; origin of anal just behind pelvic bases, and insertion separated from lower caudal origin by a narrow notch; caudal fin elongated, over a fourth of total length. No crests of enlarged denticles on the caudal margins; supraorbital crests absent from cranium. No colour pattern, uniformly dark brown.
Remarks:
This genus, which differs from the closely similar Apristurus only in lacking a first dorsal fin, has been recognized by Garman (1913), Bigelow and Schroeder (1948), and Springer (1979), although Fowler (1934, 1941), following a suggestion by Regan (1912), considered the single species unidorsate only by abnormality or injury and the genus a senior synonym of Apristurus Garman, 1913. Springer (1979) noted that Pentanchus profundicolus, known from the holotype only, also differed from Apristurus species in its very short body cavity, narrower snout, and longer anal fin base. However, I compared the holotype of P. profundicolus with the holotype and only known specimen of Apristurus herklotsi (both specimens from the Philippines) and found that these specimens were very close in numerous details, including the additional characters cited by Springer (1979) as separating Pentanchus from Apristurus. This comparison suggested that the two holotypes are conspecific, that P. profundicolus is a senior synonym of A. herklotsi, that the holotype of P. profundicolus is abnormal in lacking a first dorsal fin, and that Pentanchus is a senior synonym of Apristurus. The writer hesitates to substitute Pentanchus for the extensively used Apristurus at present, especially with the lack of further specimens of herklotsi-profundicolus catsharks from the Philippines, and tentatively retains herklotsi in Apristurus pending collection of more Philippine Apristurus.