114 
NATUHAL HISTORY. [NEW BUILDING. 
is much produced beyond the edge of the shell; they have 
only one gill on each side, no lips, and the foot truncated 
and ciliated round the edge of the radiated grooved end, 
by means of which they very quickly bury themselves in 
the mud. They have only a single large ciliated syphon. 
II. In the more aberrant orders the mantle is not pro¬ 
vided with any distinct syphons, but the lobes are either 
free behind or united and pierced with one or two holes 
for the passage of the water. 
In the two following orders the animals have oblong 
elongated shells, like the preceding, and two distinct ad¬ 
ductor muscles, leaving more or less distinct scars on the 
inner surface of the shell, which shews the places the 
muscles have occupied when the animal was alive. 
The Goniopoda have a more or less compressed angular 
foot; the mantle without any syphons. The lobes are 
united so variously and so differently in genera that are 
evidently very nearly allied, that we are led to believe 
that in this order it must afford a very secondary charac¬ 
ter. They may, however, until more is known of the 
animals, be arranged in the following families. 
In some the shells are adherent to other bodies by the 
outer surface of their valves. 
In the family Chamidce , the mantle-lobes are united, 
leaving only a small hole for the passage of the small 
elongate oblong foot. The shells are porcellanous, and 
the animals attach themselves to rocks, coral, and shells 
on the sea-shore. 
The family Etheriadcz are fluviatile, being attached to 
stones and shells in the African rivers. Their mantle- 
lobes are free, with a large quadrate foot, like the Uniones, 
and the shell is pearly and blistered internally, and covered 
with a green periostraca, which is often eroded. The 
genus Mulleria appears to have been established on a 
distorted Etheria. 
The animals of the remainder of the families live free, 
moving about from place to place, and are mostly marine. 
The family Carditidce have the mantle-lobes free, but 
united behind so as to form one or two syphonal holes. 
The shells have very oblique cardinal teeth, and the 
cartilage is external; they have no lateral teeth, and the 
periostraca is thin. 
