CONCHIFERA. 
445 
each valve, which is either straight or interrupted, the teeth of 
one valve fitting into the intermediate spaces of the other valve. 
Genus 10. — NUC UL A.—ZemarcA. 
Generic Character.— Transverse, oval-trigonal, or oblong, 
equivalve, and equilateral; no intermediate area; teeth of the 
hinge linear, numerous, and pectinated, interrupted in the middle 
by an oblique produced hollow; beaks contiguous, reflected; 
ligament marginal and partly internal. 
Section * Shells sublongitudinal. ** Shells transverse. 
Nucula margaritacea. — The Pearly Nuccjla. 
Plate LXXXIX. fig-. 6. 
Obliquely ovate, trigonal; striae minute and almost obsolete ; 
covered with a greenish epidermis ; inside silvery pearlaceous ; 
margin crenulated; and having regularly pectinated teeth. One 
half inch long. Inhabits the British seas. 
Genus 11.—P E CT UNCUL US.— Lamarck. 
Generic Character. —Orbicular, doubly convex, equivalve, 
subequilateral, and close; hinge arcuated, teeth numerous, ob¬ 
lique, serrated, alternately inserted into the opposite valves, 
middle ones obsolete ; ligament external. 
Section * No distinct longitudinal furrows. ** With raised 
longitudinal radiating ribs. 
Pectunculus glycimeris .— The Delicious Pectunculus. 
Plate LXXXIX. fig. 8. 
Suborbicular, umbones produced; finely striated transversely 
and longitudinally; covered with a villous skin, under which it 
is marked with reddish-chestnut spots or bands; inside white, 
and the margin crenulated. From two to three inches long. 
Inhabits the British and Mediterranean seas. 
Genus 12.—ARC A . — Linnaeus. 
Generic Character. —Transverse, subequivalve, inequilateral; 
beaks remote, separated by the area of the ligament; hinge 
linear, straight, without ribs at the extremities; teeth of the 
