The only action properly called a clamshell is the one in Bill's link. The Germans called it a somethingorother verschluss. As stated, a reinforced action is properly called a bolstered action.

Bolsters are very popular with gunmakers today, but are purely cosmetic and thoroughly useless. In the old days, bolstered boxlocks, like the Lancaster in Bill's link, were in the minority. The most ubiquitous of the pre-war trade built boxlocks (those from Webley, Leonard and Wilkes) never had them and, in use, those rifles were nothing if not robust. Bolsters were simply not needed. Unless the popular current argument about the relative differences in the steels is a red herring (P T Barnum never knew just how right he really was), bolsters sure as hell aren't needed on new DRs today. Just an excuse to uglify an action.
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