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

[paul monfils]

Yes, Trivia radians and Trivia solandri are very similar, and I have seen a few specimens where I couldn't decide for certain.  Unfortunately there isn't one clear cut characteristic you can use to separate them in all cases.  But in general, T. radians is broader for its length (smaller length:width ratio), more oval in outline (T. solandri tapers more anteriorly), more dorso-ventrally depressed (flattened), lighter in color, and often has a dark blotch on the top of the dorsum (as yours does).  Keen states that T. solandri has "a few less transverse ribs" than T. radians, but doesn't provide any actual counts.  In my opinion, the swollen nodes along the mantle line on the dorsum are usually more pronounced in T. solandri.  T. radians grows larger (16-22 mm usual size range, compared with 13-16 mm for T. solandri), and T. solandri ranges farther north, into California, U.S.A.  Both species are found in your locality.

You can see several specimens of each here: <!-- m --><a class="postlink" href="http://www.conchology.be/?t=65&family=TRIVIIDAE">http://www.conchology.be/?t=65&family=TRIVIIDAE</a><!-- m --> (though I believe the last specimen shown as T. radians is actually a large T. solandri).