Need help to identify Costa rica Pacific shell....beginner issue

[Argonauta]

Hi Paul, thanks a lot, really a lot, about the tips and the identification.
using your identification I started looking for these shells here and there, and I found some shell that look a lot to the Trivia radians, but are identified as Trivia solandri.
look at : <!-- m --><a class="postlink" href="http://cgi.ebay.com/19-20-mm-3-Pcs-Trivia-Solandri-Sea-Shell-SeaShell-/230625579836?pt=LH_DefaultDomain_0&hash=item35b25b133c">http://cgi.ebay.com/19-20-mm-3-Pcs-Triv ... 35b25b133c</a><!-- m -->
they look very , very similar to the specimen I have....what is the difference between these two species?
thanks again for everything.

bests