Trochus vs Astrea Snail: How to tell difference?

[GotamaHuy]

I am looking for the answer, but there isn't a clear one.

[Parapodia]

There are a number of differences, but the easiest way to tell them apart is the type of operculum: the Trochus has a dark one of protein or 'horn' like a fingernail, and the Astrea has one composed of calcareous material similar to it's shell. There are several species in each genus, so a Google image search will show multiple animals for each name.

 

Good background reading:

 

http://www.reefkeeping.com/issues/2004-05/rs/