Originally posted by knighttemplar1960:

Narrow strips of swamp usually have none. Large swamps can have zero, few, or many. If you find the swamp that has Bonemass there are always a few there.

Nah, one of the last seeds we played had a very small Bonemass swamp and absolutely zero crypts. Can say for sure the presence of Bonedaddy does not guarantee crypts.