The situation with cotypes is not as simple as Henk Mienis puts it. As Tom
Watters pointed out, "cotype" meant different things to different
researchers. To evaluate the current status of cotypes, we must know what
the author meant by "cotype," which means looking at the labels of many
additional specimens.

By this method, I was able to ascertain that Truman H. Aldrich labeled
specimens of species A as "cotypes" if he identified them as A, regardless
of whether the specimens were collected before or after the species was
formally named, and regardless of whether the specimens were collected at
the type locality or somewhere else. To be included as a primary type today,
only specimens collected before publication, and from a locality mentioned
in the paper and eligible as a type locality, can even be considered. The
result is that some of Aldrich's "cotypes" are syntypes and others are not;
each must be considered on an individual basis.

Others defined "cotype" as a synonym of "syntype" or with other meanings, so
we don't use the word anymore except in a historical context, e.g., to quote
a label.

Andrew K. Rindsberg
Geological Survey of Alabama