PLANULACEA. 229 



Lain. Entomostomata, Bl. — Descr. Sub-ovate, pyramidal, solid ; 

 spire measuring J- or ^ of the axis, consisting of few whorls ; co- 

 lumella contiguous to the axis, flat, truncated, and separated from 

 the outer lip by a short canal ; outer lip thickened and denticu- 

 lated within ; operculum horny, thin, with a terminal nucleus, — 

 Obs. This is a genus of small marine shells found in the West 

 Indies, &c. Fig. 365, P. sulcata. 



PLANE. {Planus.) Flat, planed, as the columellar lip of Pur- 

 pura, fig. 414. 



PLANORBICULAR. {Planus, flat ; orbis, an orb.) Flat and cir- 

 cular, as Ammonites, fig. 478. 



PLANORBIS. Miill. {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 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 Ampul- 

 larise are dextral shells, and the Pianorbes 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 fresh-water 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, 



