One citation led to another and the terminal find was a 1918 paper from Science titled "Concerted behavior of terrestrial mollusks" by someone named T. C. Stephens. The subject was the snail Cochlicopa lubrica of which the author had collected 125 dormant individuals from the door of a laboratory building in Iowa. The author summarized his findings and conclusions thus:
...all exhibited precisely the same orientation, viz., the apex of the shell pointed downward...It seemed to the vriter that so many of these snails being found together, and with similar orientation, was a fact inviting explanation, which, however, he is unable to furnish...And while I am unable to explain the behavior of these snails, I am inclined to look upon it as a sort of concerted action.There are 2 separate questions here: 1. Why do some species of snails aggregate? 2. Why do snails always have their apexes pointing down (or apertures pointing up) when they become dormant on a vertical or near vertical surface? As far as I know, orientation with the apex down is not specific to Cochlicopa lubrica and is displayed by all snail species regardless of shell shape (tall or wide).
To make sure things hadn't changed since Stephens' observations in 1915, I went out this afternoon and examined the Cochlicopa lubrica that have been dormant on my garage door. All 27 snails I could see had their apexes pointing down. Some snails were more or less vertical (for example, #20), while others were tilted (for example, #32), but with their apexes always below their apertures. Here are the 2 examples.
After all these years, we still don't know why some snail species tend to aggregate (there are probably multiple reasons). But I can offer an answer to the 2nd question. The pull of the gravity probably has something to do with the orientation of a snail on a surface. If a snail were dormant with its shell pointing up parallel to a vertical surface, the shell would have a tendency to flip down. This would either break the seal between the aperture and the wall and the snail would fall off or the snail would have to spend energy to keep its shell pointing up. Therefore, on a vertical surface, the apex-down orientation seems to be more natural and more economical than the opposite orientation. It follows that in the absence of gravity the orientations of dormant snails on a vertical surface should be random.
Part 4 in this series is here.