MTS are the ones that have a long, pointy, trumpet-shaped shell. Mine tend to come up out of the substrate and onto the sides of the tank, decor, etc. if I forget to turn the heater back on and the temp drops or if they are looking for food. Otherwise, they are almost always in the gravel (was going to have sand and got them in advance, sand didn't work out 'cause of mold and junk in the play sand and got gravel instead).
Try to make sure to throw in a couple algae wafers once or twice a week to be sure they aren't starving, unless you have a major algae problem.