CUCULLEA. DIMY ARIA. 93 
Genus VIII. — PECTUNCULUS. — Lamarck. 
Generic Character. -— Shell orbicular, subequilateral, with 
the valves close ; umbones near to each other, and separated 
by a narrow facet, or area; hinge semicircular ; teeth 
numerous, arcuated, oblique, serrated, placed in two rows, 
one on each side of the umbones, and are separated by 
a small triangular disk in each valve, which contains the 
ligament, those of the opposite valves alternately inserted 
between each other, and becoming nearly obsolete towards 
the umbones ; two lateral, strongly marked, distant, muscular 
impressions, which are united by an uninterrupted pallial 
impression ; ligament external. 
Pectunculus Plumstedensis, Plate VIII. fig. 25. 
The well marked characters of this genus will distinguish it from all others. 
The Pectunculi are covered with a pileous epiderntis; they inhabit the 
ocean, occupying a wide geographical range, being met with in almost all 
countries. Fossil species occur in the Calcaire-grossier, and London Clay, 
and are numerous in the Crag. 
Genus IX. — ARCA. — Linneus. 
Generic Character. — Shell transverse, or subequivalve, 
inequilateral, trapeziform, or subquadrate; slightly ventri- 
cose; some are greatly ventricose; generally angular at 
both ends of the hinge line, much rounded in some species ; 
umbones small, remote, separated by the area to which the 
external ligament is affixed: hinge line rectilinear; teeth 
numerous, small, serrated, close-set, alternately inserted in 
the opposite valves ; two lateral and distant muscular im- 
pressions, ligament external. 
Arca appendiculata, Plate VIII. fig. 13. 
The Arce are marine shells, inhabiting the coasts of all climates, and bury 
themselves in the sand, in the same manner as the Cardiae; Several species, 
such as the Arca Noe, and its congeners, are affixed to extraneous substances 
by a strong tendinous byssus; such species having an open space or commis- 
sure between the front part of the valves, for its passage. This section has 
been formed into a distinct genus under the name of Bysso-arca. 
Fossil Arce occur numerously in the Tertiary deposits, and some few are 
found in the inferior Oolite. 
Genus X. — CUCULLAA. — Lamarck. 
Generic Character. — Shell subequivalve, trapeziform, or 
subquadrate ; very ventrieose; beaks distant, separated by 
a flat, external area, on which the ligament is placed; two 
muscular impressions in each valve, of which the anterior 
