Pvn H,i I'UitlS

94 AVICULID.E.
Shell shaped like the animal, inequilateral, scaly outside
and nacreous within: beaks straight: ligament long and
narrow, mostly internal and contained in a groove.
In this distinct family we have only two genera, each
containing but a single species, which widely differs
from the other. The intermediate links are wanting at
our northern end of the chain. Elsewhere both genera
and species abound in great variety; and, in remote
periods of the earth's history, what are now called the
British seas had also their full share. In studying the
particular fauna of any district we are too apt to regard
it in an isolated point of view, instead of associating it
with the faunae of other and distant regions. No mem-
ber of this large family has been noticed as inhabiting
the north-east coast or the northern part of the western
coast of America, nor is any one enumerated in the
lists of Arctic or Scandinavian Mollusca. The shell is
composed of outer and inner layers. The outer layers
are of a fibrous texture, and consist of extremely minute
and closely-packed tubes or cells, which exhibit on
their surface irregularly hexagonal prisms ; they are
separated from each other by a deposit of unusually
thick animal matter ; and, upon being steeped for some
time in caustic potash, they easily become disintegrated
and fall asunder, resembling in that state extremely
short threads of spun glass.
compact and highly
The inner layers are more
iridescent. The surface of the
shell, both in Avicula and Pinna, appears
under the
microscope to be finely punctured, as in some species of
Lepton. The anterior adductor muscle is small, showing
a transition from the Monomyaria to the Dimyaria.
Leach included this family in the first-mentioned di-
vision.