Art. XVIII.—On the Fresh-water Lamellibranchs of New Zealand.
[Read before the Philosophical Institute of Canterbury, 13th October, 1881.]
In the second volume of Dr. Dieffenbach's “Travels in New Zealand” (London, 1843) Dr. Gray described two species of Unio, brought to England by Dr. Dieffenbach and Dr. Sinclair. These he called U. menziesii and U. aucklandica. They are distinguished by the first being high and compressed, the posterior lateral teeth crowded, the inner anterior tooth of the right valve large, thick, ovate, rugose, and the other teeth small and compressed. He also distinguished a variety which was more elongated, and rounder behind, and the posterior lateral teeth not so elevated. The second species is thick, the cardinal teeth low, blunt, oblique, and the posterior lateral teeth laminar, and far off. These species were afterwards figured by Reeve in his Conchologia Iconica.
In 1850 Mr. A. Gould published in the “Proceedings of the Boston Society of Natural History” a description of another species from Auckland, under the name of U. lutulentus, and this was afterwards figured in the “Mollusca of the United States Exploring Expedition,” and also by Reeve. I have not seen Gould's description or his figure, but, according to Reeve, the shell is rudely longitudinally plicated.
In the “Malakozoologische Blätter” for 1861 Dr. Dunker described a U. hochstetteri, brought to Germany by Dr. v. Hochstetter from the River Waikato and Lake Taupo, distinguished by being “very indistinctly subverrucose in the middle. This also is figured by Reeve, and shows a shell very much truncated behind, the length being only 1.4 times the height.
In the “Mollusca of the Voyage of the Novara” Dr. Dunker also described and figured a U. zelebori. This is rather an elongated shell, the length being twice the height, and with the cardinal teeth compressed, acute, and crenated. Having examined a large number of specimens from various parts of New Zealand I have come to the conclusion that neither the shape of the shell nor the form of the teeth can be depended upon for specific characters, indeed hardly two individuals can be found alike; and from the River Avon, at Christchurch, I have obtained many individuals, living together, which combine in various ways the characters of menziesii, aucklandica, and zelebori, and the animal in all is alike. U. lutulentus is, I think, distinct, although some specimens are difficult to distinguish from U. menziesii. I have not examined the animal. U. hochstetteri I have never seen.
Notes on the animal of U. menziesii.—Pl. II., figs. A, B, C, and D. The anal siphon is dark purple outside and yellow inside; the orifice is simple. The region of the branchial siphon has numerous cirri, some of which are dark purple, others yellow; inside the shell the margin of the mantle is speckled with yellow pigment spots. The branchiæ are yellow-brown in colour, the outer darker than the inner one. They are attached at their posterior extremities to the mantle below the anal siphon, but are free from this point to the posterior adductor. The outer branchia is attached to the mantle throughout its whole length, and is obliquely truncated anteriorly; the inner branchia is not united to the foot. The foot is large, and its anterior and lower portions, which are not covered by the branchiæ, are slate grey in colour. The labial palpi are white, speckled with yellow, their length is about twice their breadth, and for about half their length the two on each side are united posteriorly. All the nervous ganglia are white, but show nothing remarkable in shape or position. The heart makes fifteen beats a minute. The alimentary canal is remarkably simple; from the capacious stomach it runs along the lower side of the body cavity to the end, and then turns abruptly upward and forward to the stomach, from whence it ascends to the heart. A crystal-style is sometimes present. The animal is diœcious. I have found both spermatozoa and ova in the month of June.
In the Catalogue of the “Conchifera in the British Museum,“and in the “Proceedings of the Zoological Society of London” for 1854, M. Deshayes has described a Sphærium novæ-zealandiæ, which has been also figured by Reeve. It is a small species of a bluish-grey colour “abundantly irregularly transversely banded;” and is said to come from New Zealand and Australia. I have never seen a shell answering to this description, and the New Zealand habitat may perhaps be erroneous.
In the “Malakozoologische Blätter” for 1861, Dr. Dunker described a Sphærium lenticula from specimens brought to Europe by Dr. v. Hochstetter from Lakes Rotoiti and Taupo. This little shell (pl. II., fig. E) is common throughout New Zealand; but it is a Pisidium, and not a Sphærium. Usually it is nearly equilateral (fig. E.a.), but occasionally very inequilateral (E.b.). The two forms are, however, only varieties of one species, as intermediate varieties completely connecting them are found in the same locality. At least both forms and intermediate varieties live together near Christchurch. The longer side of the shell is anterior. The siphons are very short, and completely united, the margin is simple. The foot is long
and colourless. The animal is very active and moves about by means of its foot, much in the manner of a Gastropod. It lives not only in mud, but also on aquatic plants.
Pisidium novæ-zealandiæ was described by Prime in the “Proceedings of the Zoological Society of London” for 1862, and afterwards in the “Annals of the Lyceum of Natural History of New York” for 1867 with a wood-cut. This may be the same as the variety b of Pisidium lenticula, as the description agrees very well; but if so the figure, judging from a tracing I had made for me, cannot be very correct, for that represents a nearly equilateral shell with a prominent umbo. The specimens described by Mr. Prime are said to have been brought from New Zealand by Mr. Cuming, so there may be a mistake in the habitat, as I believe Mr. Cuming never visited New Zealand.
Explanation Of Plate II.
Unio menziesii, with the foot expanded. c anal siphon.
Unio menziesii, left valve and mantle removed. a Anterior adductor muscle; b posterior adductor; c anal siphon; d foot; e labial palp; f branchiæ.
Unio menziesii, left side of mantle and branchiæ removed, and the pericardium laid open. a Anterior adductor; b posterior adductor; d foot; e labial palp; f right branchia; g rectum; h heart; i cerebral ganglion; k parietosplanchnic ganglia.
Unio menziesii, body cavity laid open, showing the mouth, stomach and intestine; I pedal ganglia.
Pisidium lenticula. a Type form; b variety.