"As for partridge, it is cited by a late authority 39 as a salient example of changed meaning, along with corn and store. In England the term is applied only to the true partridge (Perdix perdix) and its nearly related varieties, but in the United States it is also used to designate the ruffed grouse (Bonasa umbellus), the common quail (Colinus virginianus) and various other tetraonoid birds. "

Diligent search failed to find this word. Stinky of Mencken
to drag in that word when it does nothing to inform reader.
I don't think it is a typo. But I'll check that.

looks like a typo:

tetraonid:

# (n.) A bird belonging to the tribe of which the genus Tetrao is the type, as the grouse, partridge, quail, and the like. Used also adjectively.

(This definition is from the 1913 Webster's Dictionary and may be outdated.)

Thanks, etaoin.