Warning - this is a "newbie" question.  This has probably come up many
times in the past, so my apologies in advance...
Why, in so many cases does the genus of a species vary from publication
to publication? I have seen it many times, but I'll use the following
example:
Take the common shield limpet  - in Abbott's North American Seashells
(1955 edition) and in the Eisenberg book (1981) it is shown as Acmaea
(Collisella) pelta with Collisella being the subgenus.  Other books such
as my more recent Audubon guide list the species as Collisella pelta.
What is happening here?  Has Collisella been elevated to genus status or
is it just a convention with some to use the subgenus as the genus?  I
study other natural history also, such as wildflowers, and have never
seen this extensive use of the subgenus before.
How would you (an experienced shell collector) label my above example in
your collection.
I have noticed lots of shifting around of families also.  Is there any
publication that lists all of the latest commonly accepted mollusc
families.  I notice this changes from publication to publication also.
Is there even any concensus?
Thanks in advance for any insight you can provide.
Nora Bryan
Calgary, Alberta
CANADA