“Arca (Lissarca), rubro-fusca, sp. nov.
(Plate IX., fig. 17.)
Pectunculus miliaris? Phil. Wiegman’s Archiv. Naturgesch. 1845, p. 56.
Testa valde inæquilateralis (umbonibus fere terminalibus), ventricosa, irregulariter subrhomboidalis, antice oblique aliquanto truncata, postice latior, arcuata, mediocriter crassa, ubique rubro-fusca epidermide luteo-olivacea tenuiterque concentric laminate amicta; area dorsalis angustissima, linearis; umbones magni, mediocriter prominentes, fere contigui; linea cardinalis in medio rectiuscula, lævis, utrinque leviter arcuata, dentibus albidis, obliquis tribus vel quatuor (posticis quam anticum levissime sinuatus; valvarum margines (præter prope sinum levissimum et ad medium lateris postici) intus fortiter denticulati.
Diam. transversa 4 mill. Alt. 2⅔. Crass. 2 ½.
Hab. – Kerguelen Island (Antarctic Exped. and Transit Exped.).
It might be thought that this remarkable little species has been described from young shells on account of their smallness, however their comparative solidity, the strong teeth on the hinge and on the margin, give them the appearance of being adult. These marginal dentations are interrupted at the posterior margin, where it is faintly sinuated, and where also the little byssus protrudes.
The hinge line is moderately straight, and between it and the dorsal line, which is slightly arcuate, there is an extremely narrow linear area. The umbones are rather tumid and project slightly above the dorsal line. From the umbo to the posterior end a little above the middle of the valves there is the faintest depression. The posterior muscular scar is sub-pear-shaped, largish and well defined, the anterior is small and indistinct, and the pallial line is simple and continuous. The hinge-ligament is central, and so small as to be scarcely traceable. Its teeth are strongly developed, especially the three or four posterior ones, which are conspicuously stronger than the anterior and more oblique.
I have quoted Philippi’s Pectunculus miliaris with a note of interrogation, as I cannot reconcile certain peculiarities in the present species with his description. At all e vents they are congeneric, and therefore P. miliaris must be placed in Lissarca, which is distinguished from Pectunculus by its transverse trapezoidal form and the subterminal position of the umbones, which in the latter genus are almost central; the teeth are fewer and the valves not radiately striated or ribbed.
L. rubro-fusca apparently differs from L. miliaris somewhat in form, the umbones not being very acute, the margins of the valves are crenulated only in certain places, and not in others, the number of teeth on the hinge-line is smaller, and a dorsal area exists although it is extremely narrow.
These are, it is true, but small distinctions, and had Philippi’s description been more copious, possibly these shells might have been referred to his species without doubt. The habitat of his species is the Straits of Magellan, and this is favourable to the identity of the two species; however, until an opportunity is offered for the comparison of authentic examples, it seems to me that it will be the safest course to apply a distinctive name to the Kerguelen Island form.
Two specimens collected by Mr. Eaton differ from those obtained by the Antarctic Expedition many years ago in being rather shorter, and wider posteriorly, in having the umbones less terminal, and the ventral margin of the valves being without denticulations; the last characteristic may be due to immature age.”
(Smith, 1979: 185-187)
