226 PINNA. 



resembling leaves; inflatus. Mart. 102. fig. 980, eurystoma. 

 Zool. 111. ii. 100. imperialis. lb. pi. 109." Sw. p. 296. 



PHYSA. Drap. A genus formed for reversed species of Limnaea, 

 Auct. Fig. 310, P. castanea. 



PHYSETER. Humph. Solarium, Lam. 



PILEOLUS. Cookson. (A little cap.) Fam. Neritacea, Lam. — 

 Descr. Patelliform, with the apex sub -central, straight. In the 

 lower disc, or under surface, the centre of which is rather raised 

 or cushion-shaped, is placed the lateral, narrow, semilunar aper- 

 ture, with the outer lip marginated and the inner lip crenulated. 

 — Obs. This interesting genus is known only in a fossil state. 

 Two species are found in the upper layer of Oolite, above the 

 Bradford clay. The spire, although internal, connects this genus 

 in some degree with Neritina. Still there is no danger of con- 

 founding them. Fig. 332, P. plicatus. 



PILEOPSIS. Lam. Capulus, Montf. 



PILLAR. The usual English name for the column which forms the 

 axis of spiral shells, around which the whorls revolve. See 

 Columella. 



PINNA. Auct. (The fin of a fish.) Fam. Mytilacea, Lam.— 

 Descr. Equivalve, inequilateral, oblique, wedge-shaped, thin, 

 horny ; umboues terminal ; hinge rectilinear, without teeth ; 

 anterior margin sinuated, to admit the passage of a byssus ; 

 posterior margin truncated, gaping ; muscular impressions two 

 in each valve ; posterior large, sub-central ; anterior small, ter- 

 minal, sometimes double. — Obs. The beautiful large shells of 

 which this genus is composed, are possessed of a large, flowing, 

 silky byssus, of which gloves and hose have been manufactured. 

 They have received their name from their resemblance to the 

 pectoral fins of some fishes. Some species attain very large 

 dimensions, and measure two feet in length. A very improbable 

 story is told with regard to animals of this genus, namely that a 

 certain small species of crab is in the habit of taking refuge from 

 its enemies in the shell of the Pinna, into which it is received 

 with great hospitality and kindness by the " blind slug," which in- 

 habits it. In return for which kindness, he occasionally goes abroad 



