Diplodon menziesi, Gray, n. subsp. lucasi.

Lake Manapouri (Stat. 35—Dredging in 60 ft.).—Three specimens; one of them may be taken as adult. Shell (figs. 2 and 3) oblong-ovate, very much compressed, thin and fragile, inequilateral, beaks low, eroded; surface with close strongly pronounced rest-marks and between them a few lines of growth, all close together and foliated at the anterior end. In the adult specimen the middle part has distinct radiate nodulous sculpture, partly V-shaped, but no such ornamentation is to be found on the young specimens. The straight dorsal margin is subparallel to the ventral margin, which is slightly sinuate; the anterior margin is angularly rounded, the posterior obliquely truncated and slightly produced. Nearly the whole of the shell is covered with a thin ferrugineous coating; the epidemis is olive-green, waxy. The ligament is small, not much raised. In the right valve the two pseudocardinals are compressed, small; the upper anterior tooth is a small, smooth lamella, the lower tooth is more elevated, conoidal, and strongly crenate; the lateral tooth is almost straight, thin and rugose at its posterior portion. In the left valve there is a rather long compressed lower anterior rugose pseudocardinal, the upper tooth is quite rudimentary; the upper lateral tooth is a little higher and more rugose posteriorly than the other. Interior bluish-white, pearly, a little blotched with olive in the umbonal cavity, where there are rather large and deep dorsal scars. The adductor-muscle scars are shallow. The young specimens are slightly winged.

Fig. 2

Fig. 3

This subspecies is nearest to the typical aucklandica, but is distinguished from it by its exceptionally compressed form, the thinness of the shell, the strongly marked and close concentric lines, the more tapering posterior margin, and the feebly developed pseudocardinals. The radiate nodulous sculpture is found in many specimens of menziesi and its subspecies. Adult specimen—Length, 45 mm.; height, 24 mm.; diam., 8 mm.

Type in my collection.

I have much pleasure in naming the subspecies in honour of its discoverer, Mr. Keith Lucas.