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. 
Avicuta,* Klein, 1753. 
Prerta. Scopoli, 1777, sec. Gray. 
RipariH (sp.). Gevers, 1787. Id. 
MarGARITIFERA (sp.). Humph., 1797. 
Anontca. 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 coste ; hinge-line rectiliear, 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. 
Myritus uirunpo. Linn. Syst. Nat., ed. 12, p. 1159 (in part). 
— — ? Poli. Test. Utr. Sic., vol. ii, p. 221, t. 32, fiz. 17, 1795. ' 
AVIcULA HIRUNDO. Turt. Brit. Biv., p. 220, pl. 16, figs. 3, 4, 1822. 
— acutyaTa. Risso. Hist. Nat. des Princ. Prod. de VEur., t. iv, p. 308, 1826. 
— Arvantica. Brown. IMllust. Conch. Gr. Brit., pl. 10*, fig. 6, 1827. 
— Anotuica. “Leach.” Id. - - pl. 31, fig. 3. 
— Tarentina. Lam. Hist. des An. S. Ver., t. vi, p- 148, 1818. 
— — Forb. and Hanl. Hist. Brit. Moll., vol. ii, p- 251, pl. 42, figs. 1—3, 
and pl. S., fig. 4, 1849. 
* Etym. dvicula, from its resemblance to a Bird’s wing. 
