Pvn H,i I'UitlS

PECTUNCULUS. 165
shaped when at rest like an axe, bnt capable when in motion
of being expanded into a discoidal form.
Shell suborbicular, convex, nearly equal-sided, porcellanous
: epidermis velvety : beaks slightly incurved, and be-
coming separated from each other in the course of growth.
ligament altogether external, very strong, occupying a vaulted
cavity at the back of the shell, composed of several bundles of
cylindrical fibres, which radiate from the beak to the hingeplate,
to the outer edge of which they are united: teeth
in a curved line but in two
laminar and continuous, arranged
distinct rows : pallial scar entire : muscular scars oval, symmetrical,
and strongly marked.
The rounded form of Pectunculus prevents its being
mistaken for any other genus of the same family, except
Limopsis ; and the substitution of a compound ligament
for a simple cartilage, besides other specialties of the
hinge-structure, offer sufficient marks by which these
can be known one from the other. Moreover
genera
Pectunculus has never been observed to produce a byssus :
but I do not consider this a distinguishing characteristic.
The nature of their habitation is the same.
The present genus has descended in an unbroken line
from the Silurian epoch to the present time. The
extent of its distribution in space is equally great. It
is prolific in species ;
but only one of them has yet been
found as far north as the Loffoden Isles. This is the
kind that inhabits our seas.
Lister was the first naturalist, since Pliny, to use the
word Pectunculus; but he applied it to most bivalve
shells. His second division of Pectunculi had a fright-
fully long adjective — Polyleptoginglymi —and comprised
Area and Pectunculus. Eight years before Lamarck
published the present genus, Poli proposed the name
Axincea for the animal. This last name has been adopted
by Oken and Gray ; but as it was founded solely on
anatomical, and therefore insufficient characters, it does