>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.
 
Recent examination of the type species of Acmaea has show it to be very
distinctive, meriting separation from almost all other limpets.  Gary
Rosenberg's Encyclopedia of Seashells gives more details and references for
extensive details.  In general, the newer references will reflect more
recent discoveries, although the quality of the reference must be kept in
mind.
 
>How would you (an experienced shell collector) label my above example in
>your collection.
 
Collisella pelta
 
>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 consensus?
 
Part of the problem lies in new discoveries.  Both closer examination of
the animals and review of old literature and type collections may result in
discoveries that show that our current classification is incorrect.
 
The other problem lies in varying opinions as to what should be called a
genus, family, etc.  Some people have broader concepts, some narrower; some
people think a particular feature is very important and others less so.
Also, there is not agreement upon the philosophy of classification.  Some
people think a taxonomic unit should include all the descendants of a
particular common ancestor whereas others will accept the possibility of
recognizing one taxon as descending from another.  For example, current
evidence suggest that the cones evolved from the turrids, so that some
turrids are actually more closely related to cones than they are to certain
other turrids.  Some people would therefore want to split the Turridae into
multiple families, one of which would be the Conidae, and the whole group
would be known as Conoidea.  The old "Turridae" would no longer have any
formal status.  Others are willing to keep the term Turridae, with the
acknowledgement that it does not include all the descendants of the
ancestral turrid because it excludes the cones.
 
Thus, updating your labels will be a continual necessity, though hopefully
they are getting more accurate each time.
 
David Campbell
 
"Old Seashells"
 
Department of Geological Sciences
CB 3315 Mitchell Hall
University of North Carolina at Chapel Hill
Chapel Hill NC 27599-3315
USA
 
919-962-0685
FAX 919-966-4519
 
"He had discovered an unknown bivalve, forming a new genus"-E. A. Poe, The
Gold Bug