88 PTEROCERAS. 
tinct from that genus. It is described by Bowdich under the 
name Megadesma, on account of its large ligament, and by 
Lamarck, under that of Galathzea, a name previously used by 
him for a genus of Crustacea. P. radiata, fig. 115. Megadesma 
appears to be the preferable name, since it has the right of pri- 
ority over Potamophila. 
POTAMOMY’A. A genus of freshwater shells resembling Cor- 
bula, and probably belonging to the Myariz. 
POTAM'S or POTA’MIDES. A genus of freshwater shells re- 
resembling Ceriruium in the characters of the aperture; but 
which may be known from that genus by the thick, horny epi- 
dermis, with which they are coated. P. muricata, fig. 377, 
(Cerithium, Sow.) We think that these shells should be placed 
near MELANIA. 
PRISODON. Schum. Hyria, &c. Auct. Fig. 144, 
PRODUCTA. (Productus, produced.) Fam. Brachiopoda, Lam. 
Descr. Equilateral, inequivalve, thick, striated; one valve gene- 
rally convex, with the margin inflected, produced; the other 
valve flat, or slightly convex, with the margin reflected; hinge 
rectilinear, transverse. Obs. The peculiarity in this genus, from 
which it derives its name, is the drawing out and overwrapping 
of the anterior margins of the valves. The genus is only known 
in a fossil state. Species occur in Mountain Limestone, and 
Transition Limestone of older date. P. depressa, fig. 206. 
PRO’TO. Defr. A fossil shellresembling Turrire.ua, but having 
a spiral band reaching to the centre of each valve. P. terebralis, 
Bl. 
PSAMMOCO'LA. (shappos, psammos, sand ; cola, inhabitant.) Bl. 
Psammosia and Psammor=a, Lam. Fam. Nymphacea, Lam. 
Descr. Transverse, oblong, slightly gaping at both ends; hinge 
with two cardinal teeth in one valve, one in the other; ligament 
supported upon a prominent fulerum; muscular impressions two 
in each valve, sub-orbicular, distant; palleal impression with a 
large sinus; epidermis thin. Obs. The genus thus described 
includes Psammotza of Lamarck, which, according to him, only 
differs in the number of teeth, and which he says are but “‘ Psam- 
mobies dégenerées.”’ The difference appears to be accidental. 
This genus differs from Tellina in not having a posterior fold in 
the margin. Fig. 100. 
PSILOSTOM’ATA. BIL. The third family of Aporobranchiata, 
Bl. containing no genera of shells. 
PTEROCERAS. Lam. (irepov, pteron, a wing; xepas, ceras, 
horns.) Fam. Ailées, Lam. Descr. Turrited, oval, ventricose, 
thick, tuberculated; spire short; aperture oval, terminating in 
a lengthened canal at both extremities; outer lip thickened, ex- 
panded, produced into horn-shaped, hollow, thickened spires, 
with an anterior sinus apart from the caudal canal. Obs. This 
