Actually, it's simply "snail" in Spanish. Snail-shell would be "cáscara del caracol". I can't imagine what the connection is. My point in posting that somewhat pompous-looking explanation about military tactics was to make a sort of oblique observation on the whole business of word-puzzles: for some people, it seems, the most important thing is just to identify the word indicated, the solution, then move on. Somehow, I think the setters of the Macmillan quiz would be mortified to learn that their efforts were in any way associated with an unseemly blog squabble. Isn't the whole point of doing such puzzles, ultimately, to expand one's knowledge? Isn't that what the setters aim for? Are they complete fools for believing that those who solve their puzzles actually spend any time at all digesting the new words they have learned? Apparently they are.