CLASS III. 
4. Nupmrancur. (1) 
No shell. Pl. 15, fig. 7, 
8, 9. 
2. INFEROBRANCHI. 
Ditto. Pl. 15, fig. 10, 
a4, 42. 
3. TEcTiIBRANCHI. 
Shell more or less de- 
veloped, within the man- 
tle. PI. 16, fig. 1, 2, 3. 
4. PULMOBRANCHI. 
A great number with 
turbinated shells, always 
without opercula. 
5. PEcTINIBRANCHI. 
Shells completely tur- 
binated, and generally 
more or less closed by 
an operculum attached to 
the posterior part of the 
foot. 
6. Scurmrancu. (2) 
Shells very open, often 
in the form of a shield, 
without opercula. Pl. 14, 
fig. 20. 
7. CYcLoBRANCHI. 
Shells of one or several 
pieces, never turbinated, 
without opercula. 
(57 ) 
ORDERS. 
Branchie naked, dor- 
sal. 
Branchis naked, un- 
der the edges of the 
mantle. 
Branchie covered by 
the mantle. - 
A cavity for respira- 
tion, opened and shut 
at will. 
Branchiz hidden in a 
dorsal cavity open above 
the head. PI. 13, fig. 18. 
Ditto. 
Branchie around the 
foot, under the edges of 
the mantle. Pl. 14, fig. 
5,6 
GASTEROPODA. 
Hermaphrodite. 
Sexes separate. 
Hermaphrodite. 
(1) The greater number swim reversed, the foot (concave like a boat) on the surface, 
aiding themselves with the edges of their mantle and using their tentacula as oars. 
(2) The heart is traversed by the rectum, and receives the blood by two auricles, 
as in the greater number of Bivalves. 
