GALLERY.]] NATURAL HISTORY. 137 
In the two following orders the animals have oblong 
elongated shells, like the preceding, and two distinct ad¬ 
ductor muscles, leaving scars on the surface of the shell. 
The Gonopoda have a more or less compressed angular 
foot; without any syphons. The mantle-lobes are united 
together 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 Etkeriadce 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 animals of the remainder of the families live free, 
moving about from place to place. 
The family Carditidce have the mantle-lobes free, but 
united together 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. 
The family of Crassinidce only differ from the former 
in the teeth being triangular and diverging like the Vene - 
ridce , and they are covered with a thick brown periostraca; 
the hinder lateral tooth of the left valve is double. 
The family of Crassatellidce are nearly similar, but 
they are at once known by the cartilage being placed in 
an internal triangular pit near the cardinal teeth. 
The family of Isocardiadce have the very oblique cardinal 
teeth and the linear external cartilage of Cardita , but the 
hinder lateral teeth of the right valve are double. 
The family of Lucinidce have an orbicular white shell, 
with none or only a few very small cardinal teeth, and the 
mantle-lobes are more or less united together beneath, 
