156 LECTURE XL 



the different genera, or sets, with some particular 

 parts or organs not to be observed in Snails ; 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 Linnaean genera of the 

 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 Urii- 

 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 Mollusca. 



The most striking deviation from the general 

 plan of Nature in the Univalve shells is exhibited 

 in the Linneean genus Argonauta or Argonaut ; 



