84 PLEIODON. 
columella contiguous to the axis, flat, truncated and separated 
from the outer lip by a short canal; outer-lip thickened and 
denticulated within; operculum horny, thin, with a terminal 
nucleus. Obs. A genus of small marine shells. Fig. 365, P. 
sulcata. 
PLANORBI'CULAR. (Planus, flat; orbis, an orb.) Flat and 
circular,as Ammonites, fig. 478. 
PLANOR BIS. (Planus, flat; orbis, an orb.) Fam. Lymnacea, 
Lam. and Bl. Descr. Thin, horny, convolute, planorbicular, 
nearly symmetrical : spire compressed, concave, consisting of nu- 
merous gradually increasing whorls, which are visible on both 
sides; aperture transversely oval, or nearly round; peritreme 
entire: outer-lip thin; inner-lip distinct, spread over a part of 
the body whorl. Obs. This is a genus of shells abounding in 
ditches and stagnant pools, which is not liable to be confounded 
with any other, excepting the discoidal species of Ampullaria, 
which may be distinguished by the aperture being broadest in 
the opposite direction. Fossil species are found in the fresh- 
water strata of the Isle of Wight, and the neighbourhood of 
Paris. Fig. 311, P.corneus. All the species are reversed shells. 
PLANORBU'LINA. D’Orb. A genus of microscopic Forami- 
nifera. 
PLANULA’CEA. Bl. The second family of Cellulacea, Bl. con- 
taining the genera Renulina and Peneroplis. 
PLANULA’RIA. Def. Peneropus, Bl. A genus of microscopic 
Foraminifera. 
PLANULI'NA. D’Orb. <A genus of microscopic Foraminifera. 
PLAN'ULITES. Lam. Discorsrrss, of the same author. A ge- 
nus of microscopic Foraminifera. 
PLAT’YLE’PAS. Leach. (marvs, platus, wide; Aemas, lepas.) 
Order. Sessile Cirripedes, Lam. Balanidea, Bl. Descr. Conical, 
depressed, consisting of six valves, each divided internally by an 
angular plate jutting from the centre; (like the buttress of a 
wall;) operculum consisting of four valves in pairs. Obs. This 
genus differs from Balanus, Coronula, &c. in the internal struc- 
ture of the valves. Blainville’s description of the genus Chth- 
alamus partly agrees with this. Fig. 19. 
PLATIRIS. Lea. (mrarvs, platus, wide; wus, iris.) A genus in- 
cluding several species of Nayades, referred to Iriprna, of Lam. 
The genus Platiris is divided into two sub-genera: Iridina, spe- 
cies which have crenulated hinge margins; I. Ovata, I. exotica, 
Spatha, Lea; those with smooth, or very slightly crenulated 
hinges, 8. rubens, 8. Solenoides, Mycetopus, D’Orb. Fig. 151. 
PLEC’TROPHORUS. Fer. (adjerpov, plectron, spur; opea, 
phoreo, to carry.) A genus consisting of one or two species of 
small testaceous appendages fixed on the posterior extremity of 
a species of slug. P. corninus, fig. 260. 
PLEIODON. Conrad. Iridina. Lam. 
