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Volume 14, 1881
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Art. XVI.—On the New Zealand Hydrobiinæ.

[Read before the Philosophical Institute of Canterbury, 3rd November, 1881.]

Plate I.

In the “Smithsonian Miscellaneous Collection,” vol. vii., and also in the “American Journal of Conchology,” vol. i., 1865, Dr. Stimpson founded a new genus—Potamopyrgus-on Melania corolla of Gould, from Banks' Peninsula, and in my “Manual of the New Zealand Mollusca,” (Wellington, 1880,) I followed Dr. v. Martens in considering this species to be the same as Melania corolla (Reeve), and Paludestrina cumingiana (Fischer). I also followed Mr. Tenison Woods in putting all our other Hydrobinæ into Moquin-Tandon's genus Bythinella. But an examination of these shells during the last year has convinced me that Gould's species is not the same as Paludestrina cumingiana, and that all our species belong to the genus Potamopyrgus. I have not seen the “Mollusca of the United States Exploring Expedition,” but I found my identification on the fact that P. cumingiana is not found in Banks' Peninsula, and that its dentition does not correspond with the description given by Dr. Stimpson of the dentition of P. corolla, while that description does agree with the dentition of P. fischeri (Dunker), which is common in Banks' Peninsula. The dentition of all the species is so much alike that all must be included in one genus, but it is necessary to alter Dr. Stimpson's diagnosis in order to include the slight differences that are found among them.

The absence of books prevents me feeling certain that all the synonyms I have given are correct, and as three out of the four species vary very much, it is possible that other naturalists may consider some of the forms to be distinct which I consider only as varieties.

Potamopyrgus, Stimpson.

Shell, ovato-conic or oval, imperforate; body whorl more than half the length of the shell; aperture ovate, the outer lip acute; peritreme continuous or discontinuous. Operculum horny, subspiral, without any internal process. Animal with the foot rather short, broadest, and slightly expanded, in front. Tentacles very long, slender, tapering and pointed. Eyes on very prominent tubercles. Dentition. Median tooth trapezoidal,

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the inferior margin more or less trilobate. First lateral broad and excavated in the middle, contracted into a long peduncle, the denticles nearly equal. Second lateral pointed at the inner extremity; the shank broad, and thickened on its outer margin. Third lateral with the inner extremity broad and rounded, constricted at its junction with the very broad shank, which is thickened on its outer margin. Number of transverse rows of teeth, 55 to 69.

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

Formula of the denticles, 7 or 9/3 or 4−3 or 4; 9 or 11; 20 to 23; 30 to 40.

The formula of the denticles differs widely from that of Bythinella, and approaches more nearly those of Stomatogyrus and Amnicola; but Potamo-pyrgus is readily distinguished from both these genera by the shape of the third lateral tooth.

P. cumingiana and P. antipodum are both ovo-viviparous, and probably the other species are the same. The species inhabit both fresh and brackish water. They are very variable in form, and the only reliable character is the dentition.

Key to the Species.

Shell with spines, whorls more or less carinated.
Spines long; first lateral tooth with 9 denticles P. cumingiana.
Spines short; first lateral tooth, with 11 denticles P. corolla.
Shell without spines, whorls rounded.
Whorls 5 or 6; basal denticles 3-3 P. antipodum.
Whorls 4; basal denticles 4-4 P. pupoides.

P. Cumingiana.
Plate I., figs. A and E.

Paludestrina cumingiana, Fischer. Jour., de Conch. viii., 1860, p. 208.

Paludestrina salleana, Fischer, 1.c. 1860, p. 209 (?).

Melania corolla, Reeve. Conch. Icon. fig. 366; not of Gould.

Shell ovate, thin, olive-brown; whorls 5½ to 6½, angulated; a row of distant curved spines on the last two or three whorls, 10 to 17 spines on the body whorl.

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Formula of denticles, 9/3−4; 9; 23 to 25; 26 to 30.

Axis, ·2 to ·23 inch; breadth, ·12 to ·1.

Habitat: The northern part of the North Island and the valley of the Waikato. I have not seen it from Wellington, nor from any part of the South Island. Fresh-water only.

The shell is very variable in shape, especially in the angle of the whorls, which is sometimes sharp, sometimes rounded, and sometimes absent altogether on the body whorl. The peritreme is usually continuous, but occasionally it is discontinuous in apparently adult shells. The lower side of the median tooth is trilobate, but the middle lobe is not conspicuously developed, and is sometimes slight.

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The specimens figured were sent me from Lake Pupake, Auckland, by Mr. T. F. Cheeseman.

P. Corolla.
Plate I, figs. B and F.

Melania corolla, Grould. Pro. Bost. Jour. ii., 1847, p.

Amnicola badia, Gould. Pro. Bost. Jour. iii., 1848, p. 75.

Hydrobia fischeri, Dunker. Mal. Blatt. viii., 1861, p. 152

Hydrobia reevei, Frauenfeld. Abh. Zool. Bot. Ges. Wien, 1862, p. 1024.

Shell ovato-conic, reddish-brown or brown, rather solid; whorls 5 to 6, angulated; a row of close short spines on the last 2 or 2½ whorls; usually from 26 to 32 spines on the body whorl. Peritreme continuous or discontinuous.

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

Formula of denticles, 9/3 or 4−3 or 4; 11; 23; 35 to 40.

Axis, .15 to .22 inch; breadth, .13 to .08.

Habitat: South Island; abundant in fresh water, Wellington.

This is a very variable shell in shape, but the whorls are always angulated. The spines are always short, but very variable in number; sometimes several are united together, and then the number on the body whorl may not be more than 8 or 10; but their compound nature is indicated by their broad flattened base, while in P. cumingiana they are always round and long. This form may be P. salleana, Fischer. The specimens figured are from the Heatheote River, near Christchurch.

The animal is white, sparingly speckled with black on the foot, body, and tentacles; more thickly speckled on the head and rostrum; a paler transverse band near the end of the rostrum.

The animal does not glide, but moves along by jerks, dragging its shell after.

Dr. Dunker agrees with Dr. v. Martens that his H. fischeri is only P. corolla with the spines rubbed off, which is no doubt true. A. badia, Gould, and H. reevei, Frauenfeld, are certainly the same as H. fischeri.

P. Antipodum.

Plate I., figs. C and G.

Amnicola antipodum, Gray. Dieffenbach's New Zealand, 1843, p. 241.

Amnicola zealandiæ, Gray, 1.c., p. 241.

Amnicola egena, Gould. Pro. Bost. Jour. iii., 1848, p. 75.

Amnicola gracilis, Gould. Un. States Ex. Exp., p. 126, fig. 150.

Hydrobia spelæa, Frauenfeld. Abh. Zool. Bot. Ges. Wien, 1862, p. 1022.

Shell ovate, blackish-brown to olive-green, rather solid; whorls 5 to 6½, rounded, smooth; aperture ovate, peritreme continuous or discontinuous.

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Formula of the denticles, 7 or 9/3−3;9; 23; 35 to 40.

Axis, .24 to .16; breadth, .12 to .09.

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Habitat: Throughout the whole of New Zealand; abundant in fresh water, extending into brackish water.

A very variable shell, but easily recognized by its smooth rounded whorls. The aperture varies from nearly a half to a third of the length of the shell. The narrow, turreted, form is P. zealandiæ, but I do not think it is distinct, as there are many intermediate varieties.

The animal is in all respects like P. corolla. The central lobe of the median tooth is considerably produced.

Our species appears to be quite distinct from the Tasmanian Paludina nigra (Q & G), the whorls in that shell, judging from the figure, being much more oblique, and the aperture nearly parallel to the axis. Hydrobia spelæa is, I think, only a small form of P. antipodum. It was found, along with H. reevei, with moa bones in the Collingwood caves near Nelson.

P. Pupoides.

Plate I., figs. D and H.

Shell minute, oval, olive-green, or brownish; whorls 4, smooth, rounded; aperture ovate, two-fifths the length of the shell, peritreme continuous.

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Formula of the denticles, 9/4−4; 9; 20; 30 to 35.

Axis, .06 to .07; breadth, .035.

Habitat: Estuary of the Avon and Heathcote rivers, in brackish water. Not found in fresh water.

This shell is very constant in its form, and very unlike the other species of the genus, but the animal and dentition at once indicate its relationship. No doubt it will be found in many other localities. The central lobe of the median tooth is considerably produced; the central denticle of the first lateral is rather larger than the others. The inner surface of the operculum has some small granules at its anterior end; these are not calcareous.

Explanation Of Plate I., Figs. A To H.

A

1. Potamopyrgus cumingiana, × 5 times.

2. Potamopyrgus cumingiana embryonic shells × 25 times.

B

1. Potamopyrgus corolla, × 5 times.

2. Potamophyragus corolla animal × 5 times.

C

1. Potamopyrgus antipodum, three varieties × 5 times.

2. Potamopyrgu antipodum operculum × 8 times.

D

Potamopyrgus pupoides, × 10 times.

E

Potamopyrgus cumingiana, teeth × 470 times.

F

Potamopyrgus corolla, median tooth × 470 times.

G

Potamopyrgus antipodum, median and first lateral teeth × 470 times.

H

Potamopyrgus pupoides, median, first and second lateral teeth × 470, and operculum from the inside × 20 times.

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New Zealard Hydrobiinæ, and a New Gerus of Rissonæ