ARCADE. 65 
which around the mouth are united by two foliaceous labia, 
accompanied on each side by a pair of short palpi that on 
their free margins are obliquely truncate. The body is dimi- 
nutive in proportion to the size of the shell. The foot is 
small, subcylindrically conical, grooved at the posterior part, 
and emits a coarse byssus, which has all the active incidents of 
that of P. pectinata. We have been assisted in this account 
by extracts from Poli’s ‘ Test. Sicil.’ 
It appears then that Avicula scarcely differs from Pinna. 
We think that on comparison of the two genera, malacologists 
will be inclined to consider that their natural position is with 
the Mytilide. 
It may be in my power to supply a fresh description of 
Pinna, but extraneous aid will be required for Avicula, which 
has not occurred to us at Exmouth, though it has been fre- 
quently captured by the trawlers on the Plymouth ground. 
ARCAD/. 
The Arcade form a family of five genera, four of which, 
Arca, Pectunculus, Nucula, and Leda, are so well characterized 
by animal distinctions, as not to require further remark ; we 
add a fifth, Galeomma, which, though an aberrant genus, we 
think we shall satisfactorily show has close relations with this 
family, wherein it is placed until the animal receives additional 
elucidation ; but this, from its rarity, may be far distant. It 
is to be regretted that near twenty years ago, when we had it 
alive for some days, we did not make better use of our oppor- 
tunities. Several observers have seen it, but, it appears, not 
with perfectly satisfactory results. Naturalists are earnestly 
imvited to look after this elegant animal, and not fail to note 
at least its external organs and habitudes. Notwithstanding 
the uncertaimty attached to this genus, we think its provi- 
sional residence in this family will become a permanent one; 
at all events, almost any position will be more satisfactory 
than its present assuciation with the Kelliade. 
I 
