236 POTAMOPHILA. 



anterior. The posterior extremity of the aperture of a spiral 

 univalve shell, is that nearest to the spire. In patelliform shells 

 the anterior and posterior extremities are distinguished by the 

 muscular impression, which is annular, enclosing a central disc 

 in the inner surface of the shell, excepting where it is interrupted 

 by the place where the head of the animal lies, which of course 

 is anterior. The posterior is marked p. in fig. 119, and 387. 

 See Anterior. 



POSTERO-BASAL MARGIN of a bivalve shell is the posterior side 

 of the margin opposite the hinge. 



POSTERO-DORSAL MARGIN is the posterior side of the hinge. 



POTAMIS or POTAMIDES. Brong. A genus of fresh-water 

 shells resembling Cerithium in the characters of the aperture, 

 but which may be known from that genus by the thick, horny 

 epidermis with which they are coated. P. muricata, fig. 377. 

 (Cerithium, Sow.) We think that these shells should be placed 

 near Melania. 



POTAMOMYA. A genus of shells resembling Corbula, in every 

 respect except that of being inhabitants of fresh-water. Fig. 

 498, 499, represents one of these fresh-water Corbulse. 



POTAMOPHILA. Sow. (UoTajjLiQ, potamis, river ; $i\iog, philios, 

 choice.) " Conques fluviatiles," Lam. — Descr: Thick, equivalve, 

 inequilateral, trigonal, covered with a greenish brown, smooth, 

 horny epidermis ; hinge thickened, broad, with one central, 

 notched cardinal tooth in one valve, and two in the other, with 

 indistinct lateral teeth ; ligament large, supported on prominent 

 fulcra ; muscular impressions two in each valve, sub-orbicular. — 

 Obs. The name given to this shell refers to its place of abode, 

 being found in rivers. It is the Venus sub-viridis of some 

 authors, although being a fresh-water shell, and having an 

 incrassated hinge, and a smooth, thick epidermis, it is most 

 distinct from that genus. It is described by Bowdich under t e 

 name Megadesma, on account of its large ligament, and by 

 Lamarck under that of Galathsea, a name previously used by him 

 for a genus of Crustacea. P. radiata, fig. 115. Megadesma 



