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. 

 RIPARI/E (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 costse ; 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 

 syphonal tubes. 



1. AVICULA TARENTINA ? Lamarck. 



MYTILUS HIRUNDO. Linn. Syst. Nat., ed. 12, p. 1159 (in part). 



? Poll, 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 1'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. 13, 

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



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



