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Diplodon menziesi, Gray (1843).
Unio menziesi, Gray, in Dieffenbach's “New Zealand,” vol. ii., 1843, p. 257. Unio aucklandica, Gray, l.c., p. 257. Unio waikarense, Colenso, Tasman. Journ. Nat. Sci., vol. ii., 1845, p. 250.
I am following C. Simpson in considering Unio aucklandica as a synonym only. On examining a rather large series of specimens from over twenty localities I tried to uphold it at least as
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a subspecies, but I had to give it up, as I had numerous specimens before me which could be either assigned to menziesi or to aucklandica. There is no doubt that when the extreme forms only are compared one would feel inclined to take them for distinct species, but so it is with many other species, as for instance with Helcioniscus tramosericus. However, it is convenient to refer to aucklandica as a form of menziesi which is but little winged, and having the dorsal and ventral margins subparallel.
Unio waikarense will be dealt with further on when describing the mussels from Lake Waikaremoana.
(1.) Lake Taupo (Stat. 9—From dredgings up to 100 ft.).—There are eight specimens, representing quite young to half-grown forms, only one being highly winged. All are distinctly radiately striate, and some of the youngest specimens show the typical beak-sculpture beautifully. It is represented by the accompanying diagram (fig. 1). One of the larger specimens is very distinctly sculptured with elongate nodules on the lower half down to the ventral margin in the region below the beaks. The interior and hinge are the same as in specimens of subspecies hochstetteri, to be described further on. The largest specimen measures—Length, 42 mm.; height, 28 mm.; diam., 13 mm.
Fig. 1
(2.) Lake Taupo (Stat. 9F—From dredgings up to 100 ft.).—The eight specimens have the same appearance as those of the last station; all of them have the outline of aucklandica, are finely radiately striate, and one clean olive-coloured specimen also shows nodulous ornamentation. Five quite young specimens have the beaks already so much eroded that no trace of the beak-sculpture is left. A few specimens have a light ferrugineous coating. The largest specimen shows—Length, 43 mm.; height, 28 mm.; diam., 13 mm.
(3.) Lake Waikaremoana (Stat. 14—Dredged in 50 ft.).—Compared with the type of Unio waikarense, Colenso, said to have been obtained in this lake, the four specimens collected at this station are much smaller, very little winged posteriorly, the dorsal margin subparallel to the ventral, darker in colour, and more solid; they are not concentrically sulcated, but only striated, and the marks of rest are much less distinct. All of them are finely radiately striated, a character always to be found in menziesi. The pseudo cardinals are typical, the upper lateral tooth in the left valve is much lower than the other, and crenate
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in all specimens. Inside bluish-white, pearly, blotched with yellowish, especially in the umbonal cavity.
(4.) Lake Waikaremoana (Stat. 37—Dredged in 15 ft.).—Four specimens of different size, showing all the characters of those from Stat. 14.
In some specimens in my collection, kindly sent to me by Mr. Elsdon Best, and collected in Lake Waikaremoana, the radiate sculpture is distinctly nodulous, sometimes V-shaped, thus approaching D. websteri, Simpson. Typical specimens of aucklandica from creeks near Auckland show the same sculpture to a most marked degree, and I consider D. websteri as a D. menziesi in which the nodulous sculpture is developed to the highest degree.
I have seen, thanks to Mr. E. Best's great kindness, a large number of Diplodon from Lake Waikaremoana, but not one approaching Colenso's type of waikarense, which is a large, thin, yellowish-olive-coloured shell, having more the appearance of an Anodonta. The Waikaremoana specimens are all much smaller, thicker, darker in colour, and less winged. Mr. A. Hamilton, Director of the Colonial Museum, to whom I spoke about it, and who has visited the locality, suggests that Colenso did not get his specimens from Lake Waikaremoana itself, but from some small lake or lagoon in the vicinity. This seems to be correct, as I obtained, again through the unremitting kindness of Mr. E. Best, a number of specimens from a lagoon near Ruatoki, Tuhoeland, and these are typical waikarense, Colenso; one of them showing the same outline and the same dimensions as the cotype in the Canterbury Museum.
An error in Colenso's diagnosis of Unio waikarense (Trans. N.Z. Inst., xiv., 169) wants correcting. He says, “Posterior slope keeled.” This, however, is not correct: the shell is not keeled. How misleading such an incorrect statement is may be guessed from Simpson's remark on the species in his “Synopsis of the Naiades,” p. 890 (footnote): “Suter thinks this is a variety of menziesi, but Colenso states that the posterior slope is keeled. If this is so it must be quite different from the species.” There is also an error in the quotation in Simpson's work, as he gives the year 1841 as the date of the publication of the Tasman. Journ. Nat. Sci. vol. ii., whereas it is 1845. Colenso says that he discovered the shell in 1841, but his description was published four years later.
I have compared typical specimens of waikarense with many specimens of menziesi from about twenty different localities, and I am unable to separate the two. In all essential characters the two agree, and there remains nothing but to make Colenso's species a synonym of D. menziesi, Gray.