BIVALVIA. 51 



wavy undulations, like those visible upon the recent shell. This species, in the recent 

 state, is one of our largest bivalves, and Montague says they are not uncommonly 

 a foot in length. The specimen to which our fragment belonged, probably did not 

 exceed half that size. The same authority states, p. 181, " We discovered a bed 

 of these shells in Salcomb Bay, in Devonshire, where they are called by the fishermen 

 French muscles or scallops. They lie on a gravelly bottom, covered with mud and 

 long sea-weeds, and are only to be got at particular times when the sea recedes further 

 than usual." This shell in its living state is of a sort of double composition, the thin 

 and broadest, or outer portion, being of a brown and somewhat horny texture, while 

 the thickened lining, or anterior portion, is of a nacreous substance, composed of 

 fibrous filaments, causing the shell in the fossil state to separate readily at that part in 

 a transverse direction ; and pieces of this ' fibrous shell' are often met with in the 

 Coralline Crag at Sutton, separating like finely attenuated glassy filaments. 



Avicula,* Klein, 1753. 



Pteria. Scopoli, 1777, sec. Gray. 

 Ripaexe (sp.). Gevers, 1787. Id. 

 Margaritifera (sp.). Humph., 1797. 

 Anonica. Oken., 1815. 

 Perlamater (sp.). Schum., 1817. 



Generic Character. Shell inequilateral, inequivalve, oblique ; upper or left valve 

 the larger or more tumid ; the lower or right valve with an opening for the passage of 

 a byssus ; surface sometimes smooth, at others ornamented with squamose appendages, 

 or furnished with radiating costae ; hinge-line rectilinear, often with the posterior 

 extremity prolonged into the form of an extended wing ; one obtuse tooth in each 

 valve ; paleal impression without a sinus ; ligament external. 



Animal triangular ; the edges of the mantle disunited, and the margins fringed 

 with small tentacles ; foot small, subcylindrical, beneath which is a byssal groove ; no 

 sy phonal tubes. 



1. Avicula tarentina ? Lamarck. 



Mytilus hirundo. Linn. Syst. Nat., ed. 12, p. 1159 (in part). 



— — ? Poli, Test. Utr. Sic, vol. ii, p. 221, t. 32, fig. 17, 1795. 

 Avicula hirundo. Turt. Brit. Biv., p. 220, pi. 16, figs. 3, 4, 1822. 



— aculeata. Risso. Hist. Nat. des Princ. Prod, de l'Eur., t. iv, p. 308, 1826. 



— Atlantica. Brown. Illust. Conch. Gr. Brit., pi. 10*, fig. 6, 1827. 



— Anglica. "Leach." Id. - - pi. 31, fig. 3. 



— Tarentina. Lam. Hist, des An. S. Ver., t. vi, p. 148, 1818. 



— — Forb. and Hani. Hist. Brit. Moll., vol. ii, p. 251, pi. 42, figs. 1—3, 



and pi. S., fig. 4, 1849. 



* Etym. Avicula, from its resemblance to a Bird's wing. 



