The large bivalve belongs to the genus Iridina, Lamarck,—a group of river mussels, of which there are nine reputed species, all belonging to the African continent. This little group has been divided into several sub-genera. That to which the new shell belongs is distinguished by its broad and deeply-wrinkled hinge-line, and is called Pleiodon by Conrad. The posterior slope of this shell is encrusted with tufa, as if there were limestone rocks in the vicinity of its habitat.
The small bivalve is a normal Unio, with finely sculptured valves.
The smaller univalve is concave beneath, and so much resembles a Nerita or Calyptræa that it would be taken for a sea-shell if its history were not well authenticated. It agrees essentially with Lithoglyphus,—a genus peculiar to the Danube; for the American shells referred to it are probably, or, I may say, certainly distinct. It agrees with the Danubian shells in the extreme obliquity of the aperture, and differs in the width of the umbilicus, which in the European species is nearly concealed by the callous columellar lip.
In the Upper Eocene Tertiaries of the Isle of Wight there are several estuary shells, forming the genus Globulus, Sow., whose affinities are uncertain, but which resemble Lithoglyphus.
The lake Tanganyika (situated in lat. 3° to 8° S. and long. 30° E.), which is several hundred miles in length and 30 to 40 in breadth, seems entirely disconnected with the region of the Danube: but the separation may not always have been so complete, for there is another great lake, Nyanza, to the northward of Tanganyika, which is believed by Speke to be the principal source of the Nile.
The other univalve is a Melania, of the sub-genus Melanella (Swainson), similar in shape to M. hollandi of S. Europe, and similar to several Eocene species of the Isle of Wight. Its colour, solidity, and tuberculated ribs give it much the appearance of a small marine whelk (Nassa); and it is found in more boisterous waters, on the shores of this great inland sea, than most of its congeners inhabit.
1. Iridina (Pleiodon) Spekii, n. sp. (Pl. XLVII. fig. 2.)
Shell oblong, ventricose, somewhat attenuated at each end: base slightly concave; epidermis chestnut brown, deepening to black at the margin; anterior slope obscurely radiated; hinge-line compressed in front and tuberculated, wider behind and deeply wrinkled.
Length 43⁄4, breadth 2, thickness 13⁄4 inches.
Testa oblonga, tumida, extremitatibus fere attenuata, basi subarcuata; epidermide castaneo-fusca, marginem versus nigricante; linea cardinali antice compressa tuberculata, postice latiore, paucis rugis arata.