336 JOURNAL, BOMBAY NATURAL HISTORY SOCIETY, Vol. VIII, 
The bivalves have very limited powers of locomotion. The oyster 
family, when first they come into existence, can swim about, but they 
soon attach themselves to rocks or stones, and then become permanently 
fixed. Other families, notably the mussels, spin what is called a byssus, 
a horny or fibrous material which passes through an aperture in the 
valves of the shell, and enables them to attach themselves more or less 
firmly to rocks or stones, but by far the greatest proportion are simple 
burrowers in sand, mud, or even stone. 
This burrowing in sand or mud is effected by means of a fleshy lobe 
or foot (it is often the shape of a foot), and the process may easily be 
watched if a live shell be placed ina tank or basin upon some sand, 
‘When a sufficient time has elapsed to calm the fears and suspicions of 
the animal, it will protrude its foot from between the valves of the shell, 
thrust it down into the sand, and then with a jerky motion the shell is 
drawn vertically downwards a little ; and the process is repeated till it 
has disappeared. The whole proceeding occupies only a very few 
minutes, but some are much more rapid in their actions than others. 
The depth the animal will burrow seems to depend upon the length 
of its siphons ; these, when it is down, are protruded until their fringed 
mouths lie exactly on the surface of the sand, and in this position they 
are quite invisible until the currents of water they attract and repel 
point to their presence. 
The only instance of activity I have seen amongst the bivalves was 
some young specimens of the family Meroé, which, after driving their 
foot into the sand, jerked it out, causing their shells to shoot a distance 
of a few inches through the water. 
We now come to our classification : 
Division I.—Shells with two muscular impressions or scars on the 
inside of the valves at opposite sides and nearly equal in size. 
Division IJ.—Shells with two unequal impressions ; but this 
division will not be further considered in this paper, as the only 
representatives of it are some specimens belonging to the well- 
known mussel family, and they are very uncommon. 
Division I1i.—Shells with one impression, or two or three close 
together and centrally placed. 
Before going further, it is necessary to note that the umbo is the 
name given to the beak or commencement of growth of the shell, and 
