Occasionally one of the tetrads takes the form of a cross (fig. 249).
In one stage these double U's look much like tetrads (fig. 138).
A whole Irish law-book is composed in the form of heptads; while triads, tetrads, &c., occur in every part of the Laws.
If there are no sarcina, but tetrads or staphylococci (rarely merismopedia), then the division is in two planes.
When such ...