Left:Perfect form of peristome, a; though much compressed, the cavity of the chamber, partially filled with clay, is well-defined<br>
Right:This specimen is also perfect, but in consequence of its position in the clay, only one-half of the basal or upper part of the margin of the peristome (a) is visible
<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