Dear Art:
That's because Siphonaria pfefferi doesn't exist. The genus Siphonaria is
in the subclass Pulmonata (is that still being recognized?) and in the family
Siphonariidae (false limpets).
I think you are looking for Siphonalia pfefferi (Sowerby 1900) which is a
buccinid (pfeifferi is the incorrect spelling). Yes, the name is valid.
-Sarah Watson