The apical portion of the phragmocone of a <i><a href="https://en.wikipedia.org/wiki/Belemnotheutis" class="extiw" title="w:Belemnotheutis">Belemnotheutis</a></i> invested with the external integument; at a, a, a, is seen the corneo-calcareous layer which lies beneath. To the right is slightly magnified to show the striated surface; a. marks a denuded space, exposing the internal layer.
<a href="https://en.wikipedia.org/wiki/Gideon_Mantell" class="extiw" title="w:Gideon Mantell">Gideon Mantell</a>
<i><a rel="nofollow" class="external text" href="https://books.google.com/books?id=tGwOAAAAIAAJ&amp;printsec=frontcover#v=onepage&amp;q&amp;f=false">Philosophical Transactions of the Royal Society of London</a></i>. For the Year MDCCCXLVIII Part I