242 PINNA. 



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

 raised or cushion- shaped, is placed the lateral, narrow, semi- 

 lunar apertme, with the outer lip marginated and the inner lip 

 crenulated.— 055. 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, con- 

 nects this genus in some degree with Neritina. StiU there is 

 no danger of confounding them. P. plicatus. PI. xv. fig. 332. 



PILEOPSIS. Lamarck. 1822. Captjitjs, Montf. 1810. 



PILIDIUM. Forbes and Ilanley. A genus established on 

 anatomical grounds for the reception of a little patelliform 

 sheU, the P. fulvum, British MoUusca, p. 441, pi. 62, fig. 6, 7. 



PILLAE. The usual EngHsh name for the column which forms 

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



CoiiTJMEIiLA. 



PIISTNA. Auct. (The fin of a fish.) i^am. Mytilacea, Lam. — 

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

 horny ; umbones 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 fabulous 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 re- 

 ceived with great hospitality and kindness by the " blind slug^^ 

 which inhabits it. In return for which kindness, he occasionally 

 goes abroad to procure food for both. On his return he knocks 

 at the shell, which is opened to receive him, and they share 

 the supplies together in convivial security ! Some species are 

 smooth, although the greater number are imbricated or crisped 

 outside. P. saccata. PL ix. fig. 162. 



