156 
LECTURE XL 
the different genera, or sets, with some particular 
parts or organs not to be observed in Snails j and 
the Snails themselves are formed on the plan of 
the genus Limax or Slug, which, as I mentioned 
at our last meeting, may be considered as the ar- 
chetype or pattern of most of the animals of the 
univalve shells. 
The animals inhabiting the bivalve shells are 
formed on a different plan, and, except in a few 
particular instances, bear a general resemblance 
to the animal of the muscle and the oyster, and 
are closely allied to the Linnsean genera of th© 
Naked Mollusca called Tethys and Ascidia. 
The animals of the Multivalve Shells vary con- 
siderably in their structure; for while some are 
shaped like the animals of the Bivalves, others are 
formed like those of the major part of the Uni- 
valves ; that is they have a snail-like shape ; and 
lastly, others are of a habit or appearance totally 
differing from any of the Univalve or Bivalve 
tribes, and peculiar to themselves and to the ge- 
nus Triton among the naked iMollusca. 
The most striking deviation from the general 
plan of Nature in the Univalve shells is exhibited 
in the Linna^an genus Argouanta or Argonaut ; 
