Hi Linda,

This is not a matter of noun vs. adjective.  The first thing to know is that
a final "i" on a species name indicates the species was named after a man
(an "ae" ending indicates it was named after a woman).  When a species is
named after a person, the specific name is formed by adding the "i" or "ae"
suffix to the person's name.  The double "i" ending can occur for a couple
of reasons.  First, if the man's name already ends in "i", then the
additional "i" added to form the specific name will result in a double "i".
A species named after Mr. Martin from the USA would be called martini, but
one named after Mr. Martini from Italy would be martinii.  Many of the
double i's seen on species names however, date back to the 18th century or
thereabout, when it was a popular custom for professional men like doctors,
lawyers, professors, scientists, etc. to latinize their surnames.  This
resulted in many names ending in "i" which otherwise would not have, and
because these individuals were prominent academics, many species were named
after them.  A few names are found both ways, for example, the frequently
seen name Sowerby.  There are quite a few species named sowerbyi, but there
are also species named after the latinized form - sowerbii; and even a few
names created by latin purists who replaced the "w" in his name with "v" -
soverbii.

Paul M.