121 
EAST. ZOOL. GAL.] NATURAL HISTORY. 
The Discinidce (Case 38), on the other hand, have the 
tendon passing out of a linear slit near the middle of the 
under valve. 
The Craniadce (Case 38) are immediately attached by 
the outer surface of their under shell. 
The fourth class of Pteropodous Mollusca (Ptero- 
poda, 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 
towards the break of day. They are all hermaphrodite, 
with vent and the orifices of generation on the right side 
of the base of the fins. Some have distinct eyes. 
The Thecosomata, or Shell-bearing Pteropodes , are so 
called because their body is inclosed in a thin shell; their 
head is indistinct, the mouth being placed in the centre of 
the two large wings, which are united into a funnel-shaped 
expansion ; their gills are internal. They use their fins as 
oars to their boat-like shell, when they swim on the calm 
ocean. 
The family of Cleodor 'xdce have an elongate or subglobose 
conical glassy shell, and the fins are simple without any 
intermediate foot-like lobe. The body is divided into 
two distinct parts, the head with the two fins, and the 
arge and swollen body. The gills are superior and internal. 
In some there are lateral slits in the sides of the shell, 
which are interrupted in front in the globular shells of 
the Hyal&ce , and continued to the mouth in the elongate 
Diacrice; in others which have an elongate tapering shell 
there are no lateral slits, as Cleodora , &c. 
The family Limacinidcs is probably allied to the 
former, but it has a spiral discoidal shell. 
The family of Cuvier'idce have a glassy conical cylin¬ 
drical shell, which becomes truncated in its adult state. 
G 
