Skull of <i><a href="//commons.wikimedia.org/wiki/Anas_crecca" title="Anas crecca">Anas crecca</a></i>
Frank E Beddard
<i>The Structure and Classification of Birds</i>: <a rel="nofollow" class="external text" href="https://www.biodiversitylibrary.org/page/984474">Fig. 224</a> (1898), Frank E. Beddard