Horugai
They are called horugai and were used for a variety of purposes including military uses like calling troops into battle and ceremonial uses like opening a festival. Lots of pacific islanders use them for these things as well. One of my friends who studies koryu and owns a couple says he read that lower-pitched tones were used before a battle to intimidate (think the horns at Helm's Deep from LOTR ) and higher pitched ones to send messages during a battle as their tones would carry over the noisy mess. But he wasn't sure about this, so who knows. He has three or four from various times and with various pitches.
Sometimes you can still find them in shops, but I won't tell you where, because I want one too!
I found this page from a brief google search and know nothing about it, but at the bottom there is a scanned pic of a ukiyo-e with a samurai using one: link As you will find out, the shells have a buddhist connection and you can find small ones used as implements.
Here's a page from Osprey Publications (Steven Turnbull...).
Can't find one for sale just now, may post a pic of authentic one from a book I have sometime...
As far as being taught usage in a koryu, well, that's beyond my knowledge.
J. Nicolaysen
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"I value the opinion much more of a grand master then I do some English professor, anyways." Well really, who wouldn't?
We're all of us just bozos on the budo bus and there's no point in looking to us for answers regarding all the deep and important issues.--M. Skoss.