116 
NATURAL HISTORY. [NEW BUILDING. 
The family of Mycetopodidee are like the latter, but the 
shell is nearly cylindrical, and widely gaping at each end. 
The foot is very long and cylindrical, and the two anterior 
muscular scars are widely separated, with the smaller one 
before the larger; the lobes of the mantle are free, and 
they have no syphons. 
The family of Trigoniadce have a thick cordate shell 
of a pearly laminar texture, with an external cartilage, 
and the hinge formed of two diverging grooved inter¬ 
locking lateral teeth. The mantle-lobes are free, with a 
large foot, having an acute hooked end, somewhat like 
the cockles, by means of which the animal leaps. 
The family of Arcadee 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 ljne, and 
a broad short foot, from the end of w r hich 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 Pogonopoba 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marine body, and then allows it to be withdrawn from the 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 Tridacnidce have the solid opake white 
