POTAMOPHILA. 87 
POLYPHE’MUS. Montf. A genus composed of species of Acha- 
tina, Auct. which have elongated apertures, short spires and an 
undulation in the outer lip. P. glans, fig. 288. 
POLYPLAXIPHORA. Bl. The second class of the sub-type 
Malentozoaria, Bl. containing the genus Chiton. 
POLYSTOMEL'LA. Lam. A genus of microscopic Foramini- 
fera. 
POLYTHALAMA’CEA. Bl. (IloAvs, many ; Oadapos, thalamos, 
chambers.) The third order of Cephalophora, Bl. divided into 
the families Orthocerata, Lituacea, Cristacea, Ammonacea, 
Nautilacea, Turbinacea, Turriculacea, all of which contain ge- 
nera of chambered shells. 
POLYTHALA’MIA. Lam. The first division of the order Ce- 
phalopoda, Lam. containing the following families of chambered, 
shells, viz. Orthocerata, Lituacea, Cristacea, Sphzerulacea, Ra- 
diolata, Nautilacea, Ammonacea. Fig. 463 to 484. 
POLYXE’NES. Montf. A genus of microscopic Foraminifera. 
PORCELLA‘NA. Adanson. Mareineuia, Auct. 
PORODRA’GUS. Montf. A genus composed of species of Belem- 
nites, placed by De Blainville in the section characterized as 
swelled near the apex, and straightened towards the base. 
POSIDO'NIA. Bronn. A genus founded on the cast of a bivalve 
shel], common on Schists from Dillemburg. 
POSTE’RIOR. (After, behind, ) The posterior part of a bivalve 
shell is that on which the ligament is placed, and from which 
the umbones generally turn, The intestinal canal of the animal 
is fixed near the ventral margin on that side, and the mouth, or 
entrance to the stomach, is placed near the dorsal margin on the 
opposite or anterior side. When the palleal impression is sinua- 
ted the sinus is on the posterior side of the shell. The posterior 
extremity of the aperture of a univalve shell, is that nearest the 
spire. ‘The posterior is marked p in fig. 119 and 387. 
POSTERO-BA’/SAL MARGIN, of a bivalve shell, is the poste- 
rior side of the margin opposite the hinge. 
POSTERO-DOR’/SAL MARGIN, is the posterior side of the 
hinge. 
POTAMOPHILA. Sow. (iorapus, potamis, river; Sidx0s, philios, 
choice.) Fam. Conques fluviatiles,’”? Lam. Descr.Thick, equi- 
valve, 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 pro- 
minent 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 freshwater shell, and having an 
incrassated hinge, and a smooth, thick, epidermis, it is most dis- 
