65 ARCAD A. 
The family of the Arcade is the first, according to our 
method, in which the adductor muscles are established im 
their pyriform shape, similar size, and lateral position: the 
departure from the muscular disposition of the Ostreade, to 
the lateral one of the great mass of the bivalves, commenced 
in Mytilus, and has become complete in this family. 
Since the above was written we have added to this family a 
sixth genus, Lepton, under which head our reasons for this 
procedure will be found. 
ARCA, Linneeus. 
A. TETRAGONA, Poli et Auct. 
A. tetragona, Brit. Moll. iu. p. 234, pl. 45. f. 9, 10; (animal) pl. P. f. 1. 
Animal elongated, thick, white; mantle open throughout 
the ventral range; its edge is plam, but the upper part of the 
posterior margin is marked, as in the Pectunculi and Pectines, 
with about 40 close-set, equidistant dark dots or ocelli, which 
become obsolete and vanish anteriorly; the margin, or area 
within the ocelli, is mottled with pale yellow flake markings, 
which are also seen, though more faintly, on the under sur- 
face. The foot is white, and when at rest is bent, but, ex- 
serted, tapers subconically. There is a deep byssal groove at 
the posterior geniculated portion, for the green filamentous 
mass, which at the pomt of msertion into the cavity is split 
into two lamin; a neighbourmg gland appears to supply a 
green glutmous fluid, from which the filaments are formed ; 
we conclude so, as whenever the byssus was extracted, the 
animal immediately produced from the fissure a flow of matter 
which almost instantly resolved itself into byssal threads; and 
it is quite certain it can free itself from a state of fixity, which 
is effected by drawing the byssus from the groove, and entirely 
deserting it. This mancuvre we have also seen performed by 
the animals of Arca lactea and Galeomma Turtoni. There is 
little doubt that all byssus-bearmg mollusca, when not con- 
fined in stony cavities, have the power of a limited locomotion 
by abandoning their moorings. 
The foot, independent of the deep fissure at the base for the 
