NATURAL HISTORY. 
83 
GALLERY.] 
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 of these animals are sometimes spun into gloves, &c., like 
silk. The Pinna have an elongated shell with a longitudinal crack 
filled with a cartilage in the middle of each valve, and Atrina are 
shorter shells without any such crack. 
The family of Aviculid^e 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 sur¬ 
face of the dorsal edge ; the animal is also destitute of the hinder ap¬ 
pendage. They live, as it were, anchored to rocks, corals, and other 
marine bodies by their byssus ; the anterior adductor muscle 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 
Vulsella 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 part of the hinge 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 ( Margarita ) 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 dis¬ 
tinct pits in the hinge margins, as in Crenatula , where the shell re¬ 
sembles the Aviculce in shape, and having a thin margin, the pits pro¬ 
ject into the cavity, or in Perna, which are like the Mallei or Hammer 
Oysters in form, but having a very thick hinge margin, forming an 
area on the top of each valve, the cartilage pit forms distinct grooves 
across it. Besides these there are numerous fossil genera, as Inoce- 
ramus, Packymya , Gervillia , which must be referred to this family ; 
the latter is peculiar for having teeth like Avicula, but more developed, 
as well as the many cartilage pits of Crenatula. The Dalacia are like 
the Crenatula , but have the umbo some distance from the front of the 
hinge margin instead of quite at the angle. 
The Micropod a have a large single adductor muscle, placed nearly 
in the centre of the sub-orbicular shell. The mantle-lobes are always 
entirely free, except near the dorsal edge, and without any peculiar 
tubes; they generally have only the rudiment of a foot on the lower 
side of the abdomen, and sometimes no apparent one. fin some the 
shells are solid, thick, and the animal has a more or less developed foot; 
the cartilage is placed in a small triangular internal pit. 
The family of Pectin hle have a moderate sized hatchet-shaped foot, 
which enables the animal to move about, and they have a tuft of byssus 
at its base, which passes out at a notch under the front margin of the 
right valve, by which they affix themselves to rocks and other marine 
bodies, like the Muscles. The dorsal edges of the valves are produced 
at each side into ears. The Pectens have small bright eyes like spots 
on the edge of the mantle, which are not found in the Lima. The 
