“Amussium meridionale, n. sp. (P1. XXIV. figs. 1-la).
Amussium lucidum, var. striata, Jeffreys, Proc. Zool. Soc. Lond., 1879, p. 562.
Testa fragilissima, paulo inæquivalvis, iridescens, pellucida, utrinque anguste hians. Valva dextra liris filiformibus numerosis radiantibus, incrementique lineis concentricis ornata, sinistra magis opaca concentrice lirata, liris tenuissimis, complanatis, sensim accrescentibus. Auriculæ subæqua1es, anticis interdum paulo majoribus. Pagina interna nitida, iridescens, liris tenuissimis undenis instructa, in valva dextra radiatim
striata, striis cum liris externis congruentibus.
Var.; Testa liris internis duodenis munita, valva dextra extus cancellata.
This species is slightly oblique, excessively thin, a little inequivalve, pellucid white, and narrowly gaping above on both sides. The right or deeper valve is more glossy than the left, and sculptured with numerous very slender radiating liræ and concentric waves and striæ of growth. The other valve is ornamented with close-set very fine concentric shallow grooves and depressed ridges. The former under the microscope exhibit a very minute reticulation, each parallel zone of this fine net-work being connected with the one above and below by elongate meshes passing over the intervening ridges. The auricles are small, at times denticulated above, slightly unequal, the anterior being rather larger than the posterior, or this proportion may be reversed. The beaks are moderately acute, the sides meeting at an angle of about 115 degrees. The valves are glossy, somewhat iridescent within, and strengthened with eleven fine liræ, the longest of which extends about two-thirds across the interior. The right valve is also very finely striated, the striæ corresponding to the slender liræ of the outer surface.
Length 14 mm., height 14, diameter 4.
Habitat.—Station 158, in the Southern Ocean, south of Australia, at a depth of 1800 fathoms. Also Station 146, Southern Ocean, east of Marion Island, in 1375 fathoms. Also Station 302, west of Patagonia, in 1450 fathoms.
The young of Amussium dalli must approach very closely to this species. It may, however, be distinguished on account of the more acute umbonal angles and the different character of the surface structure of the left valve.
The specimens from Station 302 present one or two differences from those found at the two other localities. The right or deep valve has some of the concentric lines of growth elevated into slender liræ, which, crossing those radiating from the beak, produce a distinctly cancellated surface. Besides this, the valves are strengthened with twelve instead of eleven radiating liræ. However, taking into consideration the exact similarity of the microscopic sculpture of the left valve, I believe it advisable to consider this form merely as a variety. It is most surprising to me that this species could for an instant be considered as a variety of Amussium lucidum. Any one holding such a view certainly must either be wanting in perceptive power or his ideas respecting what are usually regarded as species be very peculiar. If any two species of this genus are to be easily distinguished, those in question are they. Amussium meridionale is larger, gapes on both sides, has both valves differently sculptured, a shorter hinge-line, and the posterior auricle in the left valve is differently sculptured. In specimens from a depth of 1000 fathoms, off the Azores, identified by the late Dr. Gwyn Jeffreys as his Amussium lucidum, this ear has two or three radiating ridges crossed by strong lines of growth, the former being wanting in Amussium meridionale and the latter much finer. The Azorean examples also differ in being quite closed at the sides.”
(Smith, 1885: 316-317)
