PLANORBIS. Müll. (_Planus_, flat; _orbis_, an orb.) _Fam._ Lymnacea, Lam. and Bl.--_Descr._ Thin, horny, convolute, planorbicular, nearly symmetrical; spire compressed, concave, consisting of numerous 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 all climates in ditches and stagnant pools, 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. It is further to be remarked that the discoidal Ampullariæ are dextral shells, and the Planorbes are sinistral or reversed; and although the latter are sometimes so flat and orbicular that it is difficult to know which is the spiral side, it may nevertheless always be ascertained by a careful examination. Fossil species are found in the freshwater strata of the Isle of Wight, and the neighbourhood of Paris. Fig. 311, P. corneus. PLANORBULINA. D'Orb. A genus of microscopic Foraminifera. PLANULACEA. Bl. The second family of Cellulacea, Bl. The microscopic Foraminifera contained in this family are described as very much depressed, not spiral, chambered, cellular, and having the septa indicated by grooves on the external surface of the shell, which increase in length from the apex to the base: some of the small cellular cavities are to be seen on the margins. This family contains the genera Renulina and Peneroplis. PLANULARIA. Defr. PENEROPLIS, Montf. A genus of microscopic Foraminifera. PLANULINA. D'Orb. A genus of microscopic Foraminifera. PLANULITES. Lam. DISCORBITES of the same author. A genus of microscopic Foraminifera. PLATIRIS. Lea. ([Greek: platus], _platus_, wide; [Greek: iris], _iris_.) A genus including several species of Nayades, referred to IRIDINA, Lam. The genus Platiris is divided into two sub-genera. Iridina, species which have crenulated margins; I. Ovata, I. exotica, Spatha, Lea; those with smooth or very slightly crenulated hinges, S. rubeus, S. Solenoides, Mycetopus, D'Orb. Fig. 151. PLATYLEPAS. ([Greek: platus], _platus_, wide; [Greek: lepas], _lepas_, rock.) _Order._ Sessile Cirripedes, Lam. _Fam._ 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 structure of the valves. De Blainville's description of Chthalamus partly agrees with this. Fig. 19.