Hmmm... At first sight it's tricky!

It seems costulata has numerous fine spiral striae, that I can't see
there, but which are rather obvious in juvenile specimens:

http://www.naturamediterraneo.com/forum/topic.asp?TOPIC_ID=19761

Adult shells progressively loose this spiral sculpture:

http://www.naturamediterraneo.com/forum/topic.asp?TOPIC_ID=26999


Thank you, friends, for this help.

Olivier

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