Right you are, Paul. Well, almost completely right. It's masculine and
feminine -ensis, but neuter -ense. A similar situation holds for other
Latin adjectives ending in -is: masculine -is, feminine -is, neuter -e. So,
for instance, these species names should be corrected to end in -ense:
 
Cardium guanacastensis
Cerithium erythraeensis
Bittium vancouverensis
 
Latin has some advantages. It doesn't change with next year's slang. It was
a formerly universal language in western Europe, so it's not hard to find a
dictionary. The zoologists and botanists of yesteryear developed it into a
well-honed tool for describing the shapes of living things. And it's a much
simpler and more consistent language than some others; for instance, it is
pronounced the way it is spelled (at least in principle). But zoologists
are becoming increasingly impatient with the irregularities, and many
proposals have been made to reform and simplify the rules. So far, Latin
still rules nomenclature, irregularities and all; but that may be so for
very much longer.
 
Andrew K. Rindsberg
Geological Survey of Alabama