Which date is to be considered for priority of a name (described in a book) : publishing date or releasing date?

I am trying to determine whether Mecomera brunnea Audinet-Serville 1838 (1839) has priority over Mecomera plana (Illiger, 1838).

The first name was published in Audinet-Serville, Histoire Naturelle des Insectes which is dated from 1839 or 1838 (see http://www.biodiversitylibrary.org/item/54314#page/4/mode/1up)

The second name was published in Handbuch für Entomologie bei Bürmeister which is also dated from 1838.

I can't find any information about the month of releasing of both books, to determine which name has priority.

Many thanks for your help,