NATURAL HISTORY. 
143 
GALLERY.]] 
which is obliquely prolonged as the shell enlarges. These 
shells become gradually moulded to the surface they rest 
on. Thus, if the shell is found on a Pecten , it is ribbed, 
and if on the spine of an Echinus, or the stem of a sea¬ 
weed, it is compressed and subcylindrical. 
The three remaining Classes of Mollusca have none or 
only a rudimentary foot. 
The Brachiopodous Mollusca (Brachiopoda) are in¬ 
closed by two regular shelly valves, one placed on the 
back and the other on the lower surface of the body, 
which are quite free from each other, or only united by 
interlocking teeth on the hinge margin. They have no 
distinct head, but the mouth is placed on the hinder 
part of the cavity between the leaves of the mantle, and is 
furnished with two long spirally twisted arms, by which 
they reach their food; the organs of respiration are 
placed on the edge of the mantle. All these shells are 
attached to marine bodies. 
The family of Terehratulidce are regular, and somewhat 
like a Grecian lamp in form, and have therefore been 
called Lamp-shells. The valves are articulated together, 
and are attached by means of a tendinous band, which 
passes out of the hole in the apex of the upper valve, 
as in the Terebratulce and Spirifer. 
The Lingulidce are attached by a tendinous tube, re¬ 
sembling the stem of the Barnacles, which projects between 
the apex of the gaping valves. 
The Discinidce , on the other hand, have the tendon 
passing out of a linear slit near the middle of the under 
valve. 
The Craniadce are immediately attached by the outer 
surface of their under shell. 
The Pteropodous Mollusca (Pteropoda) live float¬ 
ing in the sea; they are furnished with one or rarely two 
wing-like fins placed on each side of the mouth; some few 
have a small flat space, the rudiment of the foot, between 
the base of the fins. The body is soft and transparent, 
and most of them are contained in a very thin, transparent, 
more or less conical, glass-like shell; they appear on the 
surface of the ocean when it is calm, especially in the 
evening, and from the brilliancy of their colours may be 
