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Diplodon menziesi, Gray, subsp. hochstetteri, Dunker (1862).
Unio hochstetteri, Dunker, Malak. Blätter, vol. viii., 1862, p. 153.
The type was collected by Hochstetter in the Waikato River.
(1.) Lake Taupo (Stat. 46—From dredgings in 10–30 ft.).—Six specimens, blackish-brown, much corroded round the beaks, most of them more produced and rounded posteriorly than the type, but some have the posterior end distinctly truncated and more or less biangular. In correspondence with Mr. C. T. Simpson I expressed the opinion that hochstetteri is a pathological subspecies,* having seen the same form amongst specimens of rugatus, Hutton, from the Kopuaranga River,† and also amongst menziesi from the River Avon. The young shells are invariably typical D. menziesi, but on growing larger the deformity constituting hochstetteri becomes more and more apparent. Some specimens are more affected, others less, thus producing a strongly truncated biangular posterior margin, or it remains only flatly rounded. At the posterior end the periostracum is produced in thick, foliated layers, and the inner margin is considerably thickened by pearly substance, forming large rugosities, and very often pearls adhering to the shell are met with. Loose small pearls of irregular form are only exceptionally found. In my opinion the cause of this is most likely some parasitic creature, as is the case in most of the pearl-producing bivalves. The outer exposed layers round the beaks are smooth, light-brown, waxy. The concentric striation is rather coarse, the marks of rest distinct and elevated. The inside is but little iridescent, except along the ventral margin, outside the mantle impression, grey to light-brown, sometimes blotched with brown; there are more or less considerable rugosities beyond those at the posterior margin. The dorsal scars in the umbonal cavity are small and deep, and to the number of two to four. The anterior adductor impressions are irregular in shape, and much deeper than the posterior ones, which are oval and shallow. Right valve with two pseudocardinals, the upper anterior forming a small lamella, the posterior being strong, compressed, high, triangular, and slightly corrugated. The single lateral tooth is regularly slightly curved and somewhat crenate at its posterior end, which is abruptly descending. Left valve with two pseudocardinals arising from a common base, both blunt, the anterior tooth larger and but slightly crenate. The two laterals are also curved, the lower of them is strongly lamellated and denticulated at its posterior end, which slopes down very gradually.
[Footnote] * C. T. Simpson, “Synopsis of the Naiades,” p. 889 (footnote)
[Footnote] † Trans. N.Z. Inst., xxiv., 275.
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(2.) Lake Rotoiti (Stat. 8—Obtained by dredging in 12 ft.).—Two specimens of medium size, strongly concentrically ridged, umbones much eroded, posterior end subtruncated, not yet distinctly biangular. They represent the intermediate stage between the young menziesi and the full-grown hochstetteri. Interior olive-bluish, pearly, a row of small dorsal scars in the umbonal cavity, parallel to the hinge line. Muscular scars strongly impressed. Right valve with two very unequal pseudo-cardinals, the anterior small, lamellar, the posterior compressed, with a broad posterior base, crenulated, trifid; the lateral tooth slightly curved, narrow, high, truncate and corrugate posteriorly. Left valve with the two pseudocardinals coalescent, separated only by a groove, the anterior strong, triangular, rugose. The two laterals with crenate edges, obliquely truncated behind. The dimensions of the two specimens are—Length, 50 mm.; height, 34 mm.; diam., 16 mm.; and, length, 41 mm.; height, 29 mm.; diam., 12.5 mm.
Diplodon menziesi, Gray, subsp. rugata, Hutton (1883).
Unio rugatus, Hutton, N.Z. Journ. Sci., vol. i., 1883, p. 478.
Lake Waikare (Stat. 41—From dredgings in all parts of the lake; most common on sand and on stony shore).—Six specimens, two of which are quite young. The latter are winged posteriorly, the beaks already eroded, and there are three to six nodulous ridges descending in front of the umbo, nearly parallel to the anterior margin; they are concentrically finely thread-striated; colour olive-brown; the inside is bluish-pearly, yellow under the beaks. The larger specimens are subventricose and thin, less winged, the dorsal margin nearly parallel to the length-axis; the beaks are corroded, the anterior margin rounded, sometimes slightly truncate, the posterior margin produced and the ventral margin broadly convex. The concentric striae are irregular, rugose posteriorly, fine and more regular at the anterior end; the marks of rest are fairly distinct. Most of the specimens are partly covered with a dark-brown ferrugineous coating. Interior bluish, yellowish, or purple pearly, sometimes strongly blotched with olive. Muscular scars shallow. Right valve with two compressed triangular pseudocardinals close together, the anterior tooth smaller, lamellar and smooth, the posterior stronger, broader, and somewhat rugose. The lateral tooth is slender, curved, rugose on the edge of the posterior part. Left valve with two elongated, rounded, rugose pseudocardinals which sometimes coalesce, when only a slight notch indicates the original two teeth. The two laterals are long, slender, sinuate, and distinctly pectinate at the posterior edges. There are always a few small rather deep dorsal scars in the umbonal cavity.