From an amateur,
 
The label is according to the latest published data OR who you believe.
Accepting that Goto and Anseeuw are correct, then the two shells are
forms of the same species and would share the same author's name.  One
would just have the added "forma gemma" if you like or a simple "(form
gemma)."  The key is to use what works for you as there are no name cops
who will check on how a form is designated.  By the way, if you have
this shell - congratulations.  The pleurotomaria are one of those
families where I will have to settle for a representative sample (thanks
to Brian Hayes) as most remain beyond my shell budget.
 
Tom Eichhorst in New Mexico, USA
 
Craig Caddigan wrote:
 
>   A question from one of the quiet ones.
>
>   In Goto's & Anseeuw book "The Living Pleurotomariidae"
>   they say that Perotrochus gemma is a form of Perotrochus
>   quoyanus and not a seperate species. My question is:
>
>   Is this accepted as a fact, and if so how is it scientifically
>   identified?
>
>       A. Perotrochus quoyanus gemma
>             Fischer & Bernardi, 1856
>                      or
>                Bayer, 1965
>
>      B. Perotrochus quoyanus forma gemma
>            Fischer & Bernardi , 1856
>                      or
>                  Bayer ,1965
>
>      Or are they still considered 2 seperate species.
>
>                                                   Craig Caddigan