GALLERY.^] NATURAL HISTORY. 139 
The family of Arcades are all marine, and have the 
mantle-lobes free like the former; the foot is com¬ 
pressed, rather variable in shape, but always bifid, and 
furnished with a broad lower edge. The hinge of the 
valves consists of a number of transverse interlocking 
teeth, which appear to be formed by the subdivision of 
two elongate lateral teeth. In general the cartilage is 
external, arising from diverging angular lines marked 
upon the facet, formed by the gradual thickening of the 
dorsal edges, which causes the umbones to be separated 
from each other as the shell enlarges, as the Pectunculi , 
which have an orbicular shell, a lunate foot, and the 
teeth in an arched line, and the Arcce , which have an an¬ 
gular elongated shell with the teeth in a straight line, and 
a broad short foot, from the end of which the animal 
secretes a quantity of mucus which hardens into lamellae, 
and by means of which it affixes itself to marine bodies. 
The Nuculce are free, like the Pectunculi , but the teeth 
of the hinge form an angular line with the cartilages in a 
triangular pit at its angle ; they are pearly within. 
The Pygonopoda attach themselves to rocks and other 
bodies by a bundle of fibres which arise from the front of 
the base of their foot. These fibres are separately formed 
in a groove in the front of that member, and after each 
has become of a certain consistence, the animal, by extend¬ 
ing its foot, attaches the dilated end of the fibre to some 
marine body, and then allows it to be withdrawn from the 
groove. New fibres are formed as they are required 
either by the breaking of the old ones or by the enlarged 
size and greater strength of the animal. 
The family of Tridacnidee have the solid opake white 
shell, and the broad and subquadrate foot of the preced¬ 
ing families ; but they are very peculiar for having the 
mantle-lobes united together, so as to leave only three 
apertures, and for the animal being so placed in the shell 
that the hinder adductor muscle is in the middle of the 
lower margin; the hinge is furnished with very oblique 
interlocking teeth. They are the giants among the Con - 
chifera , and live attached by their byssus to rocks, shells, 
and corals. They also have the faculty of forming holes 
in the surface of the shell to which they may happen to 
be fixed. 
