"-ense" is the neuter ending.  It is used following genus names that are
neuter nouns (typically ending in "-um" - Cerithium, Cymatium, Phalium,
Vasum, etc.).  "-ensis" is the ending if the genus name is either feminine
(typically ending in "-a"  - Oliva, Cypraea, Cancellaria, Terebra) OR
masculine (typically ending in "-us" - Conus, Cantharus, Fusinus, Strombus).
There are other, less frequent feminine endings besides "-a" and other
masculine endings besides "-us".  "-ensis" would also be used in those
cases.
Paul M.