Hi Tom, et al,

I don't have the exact answer but I imagine it has to do with the animal
structure more than the shell.

There have been some changes in Generic placement of the freshwater nerites
mentioned.  A. Haynes (1988) revised this group.  Ref: "Notes of the stream
neritids (Gastropoda; Prosobranchia) of Oceania".  Micronesia, 21:93-102.  I
haven't gotten a copy of this yet.

granosa is now under Neritina (along with vespertina)
cariosus is now under Clithon (along with neglectus)

Kay (Hawaiian Marine Shells, 1979) listed vespertinus, cariosus and
neglectus under Theodoxus.  I've changed my copy of her book to reflect
these changes.

The reference I used is:
"Species Profiles: Life Histories and Environmental Requirements of Coastal
Vertebrates and Invertebrates Pacific Ocean Region, Report 3, Amphidromus
Macrofauna of Hawaiian Island Streams, Technical Report EL-89-10, US Army
Engineer Waterways Experiment Station, Vicksburg, MS" by Robert A. Kinzie
III, August 1990.
This report is available free from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161

I don't have an e-mail but Kinzie's address is (if still valid):
Hawaii Insitute of Marine Biology
University of Hawaii at Manoa
PO Box 1346, Coconut Island
Kaneohe, HI 96744

John

John & Cheryl Jacobs
Seffner, FL
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----- Original Message -----
Thomas E. Eichhorst wrote:

I have a couple of similar looking nerites, Clypeolum granosa and Theodoxus
cariosus.
Both are from Hawaii and very similar in structure.  Anyone know why the
genera assigned are different?