118 
NATURAL HISTORY. [NEW BUILDING. 
bearded. They differ from the Micropoda chiefly in 
having two more or less unequal adductor muscles. 
The family of Pinnidce have a large triangular shell of 
a prismatic crystalline texture, united by a linear mar¬ 
ginal cartilage, the apical part of the valves is divided in 
half by a central longitudinal suture filled with a cartila¬ 
ginous substance. The animal has two double lips besides 
the usual pair of appendages, and by the side of the vent, 
which is above the large hinder muscle, there is a conical 
contractile appendage, the use of which is unknown. 
They live sunk in the sand, or between cracks in rocks, 
with their gaping truncated end just above the surface. 
The beards ot these animals are sometimes spun into 
gloves, &c., like silk. 
The family of Aviculidce differ from the former in 
having a notch in the front margin of the right valves for 
the passage of the byssus or beard, and in the cartilage being 
placed in one or more pits in the surface of the dorsal edge; 
the animal is also destitute of the hinder appendage. 
They live, as it were, anchored to rocks, corals, and other 
marine bodies by their byssus ; the anterior adductor mus¬ 
cle is small and rudimentary, and the hinder one larger, 
and nearly in the centre of the cavity, leaving a large 
scar near the centre of the disk of the shell. In some 
the cartilage is placed in a single cartilage pit under the 
umbo. Among these the Hammer Oyster ( Malleus ) is 
peculiar for the dorsal edge being elongated, and the disk, 
which is very narrow, being much produced, so that the 
shell assumes the form of a hammer. The Vulsellcz that 
live sunk in the surface of sponges are very variable in 
shape, but the cartilage pit is large and always produced 
into the cavity of the shell. Reniella is only a distorted 
specimen of the common species of Vulsella. The Aviculce 
are ovate, convex shells, with the hinder margin produced 
so as to resemble the forked tail of a bird hence 
they have been called Swallow-tailed Muscles. The 
mother-of-pearl shell only differs from these in the disk 
of the shell being larger and rounder, and the dorsal 
edges less produced. In the other genera the cartilages 
are placed in several distinct pits in the hinge mar¬ 
gins, as in Crenatida , where the shell resembles the Avi - 
