GALLERY.]] NATURAL HISTORY. 141 
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. In some the car¬ 
tilage 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 Vulsellce that live 
sunk in the surface of sponges are very variable in shape, 
but the cartilage pit is large and always produced into the 
eavity 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 Crenatula, where the shell resembles the Avi¬ 
culce in shape, and having a thin margin, the pits project 
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 car¬ 
tilage pit forms distinct grooves across it. Besides these 
there are numerous fossil genera, as Inoceramus , Pachy - 
my a, 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 Micropoda 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 have 
none or only the rudiment of a foot on the lower side of 
the abdomen. 
In some the shells are solid, thick, and the animal has 
