252 POTAMOPHILA. 



bones ; and by fcbe sinus (when there is one) in the palleal 

 impression, which is always near the posterior muscular impres- 

 sion. In some shells, however, it is very difficult for a learner 

 to trace these marks ; such bivalves, for instance, as have the 

 ligament spread out on both sides of the umbones ; such as are 

 nearly symmetrical, and have the umbones consequently straight, 

 and a single muscular impression near the centre of the valv^. 

 The Brachiopodous bivalves have a different position, with rela- 

 tion to the animal, from the other bivalves, so that the hinge line 



. is the posterior extremity, and the part where the. valves open, 

 is the 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 in- 

 terrupted 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 Akteeioe. 



POSTEEO-BASAL MAEQIN of a bivalve shell is the posterior 

 side of the margin opposite the hinge. 



POSTEEO-DOESAL MAEQIN 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. (Cerithium, Sow.) We 



. think that these shells should be placed near MeIiAjsta. P. 

 muricata. PL xvii. fig. 377. 



POTAMOMTA. A genus of shells resembling Corbtda, in every 

 respect, except that of being inhabitants of fresh-water. Pig. 

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



POTAMOPHILA. Sow. (YioTafiiQ,potamis,xiYQV, <^i\io£,philios, 

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

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

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



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



