450 
CONCHIFERA. 
This genus is divided into three sections.—* The anterior 
primary tooth, with a striated cleft, or the margin dentated.— 
** Anterior primary tooth not striated in its cleft, nor dentated 
in the margin.—*** The internal margins crenated or dentated. 
Genus 23 .—EXOLETA.— Brown. 
Generic Character. —Shell strong, orbicular, beaks much 
turned to one side, beneath which is a short cordiform impres¬ 
sion ; hinge with three primary teeth, two of which are contigu¬ 
ous, and the other divergent, which is broad in the right valve, 
and cleft in the centre, to receive that of the opposite valve, 
which is slender; cartilage situated within the outer surface of the 
shell; margins very broad, and somewhat concave; edges very 
thick. 
Exoleta orhiculata —The Orbicular Exoleta. 
Tlate LXXXIX. fig. 23. 
Orbicular, strong, with regular coarse, close set transverse 
striie; generally white or pale brown, with sometimes three or 
four brown radiations emanating from the umbo, and terminating 
in the margin ; inside white. Two inches long. Inhabits the 
European seas. 
Genus 24. — GYP RINA.— Lamarck. 
Generic Character. — Shell equivalve, inequilateral, obliquely 
heart-shaped, beaks obliquely bent; hinge with three unequal 
teeth, approximate at the base, and slightly divergent above; 
lateral cardinal tooth remote, which is sometimes obsolete; liga¬ 
ment exterior, and partly sunk between the beaks. 
Cyprina Islandica.— The Islanjdic Cyprina. 
Plate LXXXIX. fig. 22. 
Suborbicular, convex, strong, irregularly striated; covered 
with a deep black brown epidermis; white within. Three and 
three-fourth inches long. Inhabits the Atlantic ocean and Bri¬ 
tish seas. 
Genus 25. —L A SiE A. —Leach. 
Generic Character. — Equivalve, inequilateral; hinge not quite 
central; umbo prominent; hinge with two nearly obsolete pri- 
