NATURAL HISTORY. 
GALLERY.] 
* 85 
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 Moliusca have no foot or only a 
rudimentary one. 
The third class of Brachiopodous Mollusc a (Brachiopoda, 
Case 38) are inclosed 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. 
Some are attached by means of a tendinous cord, which passes out 
between or in a groove in one of the valves. 
The family of Lingulidje are attached by a tendinous tube, re¬ 
sembling the stem of the Barnacles, which projects between the apex of 
the gaping valves. 
The family of Terebratulid^: are regular, and somewhat like a 
Grecian lamp in form, and have therefore been called Lamp-shells. 
The valves are articulated together, and the animals 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 Spirifei'. 
The family of Discinulx (Case 38), on the other hand, have the 
tendon passing out of a linear slit near the middle of the under valve, 
the shell is suborbicular and the upper valve conical like a Patella, 
but more symmetrical. 
Others are immediately attached by the outer surface of their under 
shell. 
The family of Craniad^e are attached by their flat lower valve, 
which has an oblique facet on the upper side ; the upper valve is sub- 
orbicular, conical, with a subcentral apex like the Discince; the muscular 
scars of the lower valves bear some resemblance to a face, hence the 
name of Crania. 
The family of Thecideidje are fossil shells, very like the last, 
but only attached by the apex of the lower valves, which is produced 
and somewhat lamp-like, and the cavity is furnished with a complicated 
apparatus to support the internal organs, as in Terebratulidce. 
The family of Productim: are fossils, probably allied to the latter; 
one valve is concave and the other flat, or concave and pressed into the 
cavity of the other; the hinge line is straight and the shell subsym- 
metrical. 
The fourth class of Pteropodous Mollusc a ( Pteropoda, Case 
38) live floating 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, shewing the viscera through the skin, 
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 compared to the evening-flying Lepidoptera. 
They are most abundant near midnight, and gradually disappear 
