252 PROCEEDINGS OF THE GEOLOGICAL SOCIETY.
depressed ; valves slightly convex ; hinge-line on anterior side extended
and rather rounded ; margin of the valves regularly rounded ;
surface ornamented by very close but distinct longitudinal costae.
This shell may be distinguished from the L. anomala by the costae being finer; and although it is larger, the anterior hinge-line is less extended. Its surface, under the lens, presents a wrinkled appearance, probably indicating the presence of fine concentric lines of growth which have been obliterated. It exhibits broad concentric bands.
Loc. Wollumbilla.
41. Mactra trigonalis, sp. n. Pl. XIV. fig. 6.
Shell small, thin, trigonal ; umbones rather anterior ; anterior end rounded and rather angular ; front margin rounded ; the surface is marked by faint transverse lines.
A thin slab from the Downs on the Nive river, appears to be almost composed of this little shell. One species of the genus is recorded by Professor M'Coy from Carboniferous strata, and another is described from the Greensand ; otherwise the genus is chiefly met with in Tertiary deposits.
42. Mytilus rugo-costatus, sp. n. Pl.L XIII. fig. 2.
Shell thick, ovately oblong; umbones terminal and acute ; posterior margin oblique and curved ; anterior side produced and somewhat rounded ; the dorsal surface is obtusely keeled, and towards the umbo has very coarse irregular concentric striae or lines of growth, which become finer towards the ventral margin.
Unlike the Mytilidoe generally, whose shells are usually thin and fragile, this species possesses a very thick test, especially in the adult examples. When a portion of the test has adhered to the matrix, leaving an inner layer exposed on the shell, the striae are not seen, and the surface appears smooth, giving it the appearance of an entirely different species.
It is from Wollumbilla, and appears to be frequent.
43. Mytilus planus, sp. n. Pl. XIII. fig. 3.
Shell small, thin, ovate, compressed, nearly equilateral ; umbones terminal and acute, ventral margin rounded ; dorsal surface smooth, with depressed concentric lines of growth, crossed by very fine radiating lines, which wholly cover the exterior. Its flattened form, with its distinct ornamentation, readily distinguishes the species.
I have only seen one specimen, which is from Wollumbilla.
44. Mytilus inflatus, sp. n. Pl. XIII. fig. 4.
Shell smooth, slightly inequivalve, convex, both valves inflated ; margins close-set ; umbones terminal, acute, anterior ; hinge-line extended and oblique ; posterior margin and front rounded ; dorsal surface smooth, with irregular concentric bands of growth.
This pretty little shell is to be distinguished by its very inflated appearance, its more extended hinge-line, and terminal umbones. Its test still retains some colour,
