As P. jenkinsi:
Diagnostic characters
Shell in general like that of Hydrobia ventrosa but with less swollen whorls and usually a larger last whorl. Aperture rounded adapically. its broadest part below the middle of its height. In some shells a keel, occasionally with projecting periostracal bristles (or bristles alone), runs spirally round some or all whorls. No right pallial tentacle. Snout with anterior white transverse band, rest dark.
Other characters
The apical angle averages about 55°. The umbilicus is usually closed. The shell has a reddish horn colour, but is often more or less completely blackened by deposits. Up to 5mm high, 2-3mm broad; last whorl occupies 65-70% of shell height, aperture about 40%.
The tentacles are white basally, anterior to the eye, blue-grey elsewhere, though the white sometimes extends along them as a narrow central line. The sides of the foot are grey, with white along the edge of the sole.
P. jenkinsi lives in water of salinity of 0-16%o and may be found alongside Hydrobia ventrosa (p. 190) in brackish situations. It is also to be found in all kinds of freshwater habitats, preferring running water to stagnant, but not apparently affected by hardness. It lives under stones, on the substratum, and on such plants as Lemna, Elodea and Potamogeton, on the epiphytic growths of which it feeds. It can take larger particles than any of the Hydrobia species, feeding best on particles 80-160 in size (Lopez & Kofoed, 1980).
Most populations of this snail contain only females, which reproduce parth-enogenetically and ovoviviparously. In some places (North Wales for example) males occur to the extent of 3-11% of the total population (Wallace. 1986), but their role is still not definite. Keeled shells are more frequent in brackish than in freshwater populations, but no adequate explanation, genetical or environmental, of the occurrence of a keel has yet been offered.
P. jenkinsi is found throughout Europe except for some Mediterranean and Balkan areas. In the British Isles it is scarce or absent in mid-Wales and in much of Scotland outside the central valley.

Shell small, ovate, acute, sometimes rimate, generally covered with a brown earthy coat, smooth. Colour dark brown. Spire elevated conic, higher than the aperture, outlines straight. Protoconch small, acute, very often eroded. Whorls 7, regularly increasing, flatly convex ; base convex. Suture not much impressed. Aperture ovate, angled above. Peristome continuous, black, with a white inner callus, Columella arcuate, very little rerlexed. Operculum brown, horny, paucispiral; nucleus subcentral.
Diameter, 3,2 mm. ; height, 6,1 mm.
Dentition.— Hutton, T.N.Z.I., xiv, 145, pl. 1, f. G.
Type in the British Museum.
Hab.—Throughout New Zealand, very often in brackish water.
Remark.—The statement made by Von Martens that " some specimens are bristly " is not correct. This species has never been found with spines or a carina.