68 
“eaten by the rails and other water birds, who often break the 
operculum, the only vulnerable part, to get at the animal.’””* 
From what I have said, we are now able to conclude pretty 
accurately that paleontologists have scarcely any knowledge of 
the subject of our discussion. Notwithstanding, its value in 
the classification of certain of the extinct species must unques- 
tionably be of no slight aid. In one great class, for instance, 
of the fossil Cephalopoda, the Ammonitide, this appendage, 
known as the Aptychus and Anaptychus,t when taken in 
connection with the form of the mouth and other natural 
features, is now relied on with the utmost confidence as a trust- 
worthy ground of new generic distinctions by such authorities 
as Wrieut, Surss, Zrrren, Waacen, and Von Mosstsovics; so 
that the present classification of the Ammonites, which is a 
mass of confusion, based, it is true, upon a natural, though, 
taken alone, an insufficient character—that of the late Baron 
Lxeorotp Von Bucn’s configuration of the sutures—is being 
superseded, having clearly had its day, since even now we see 
it begin to fade into the limbo of past failures. There is, 
however, no denying that the difficulty is to satisfactorily 
prove the connection between the valves referred to and their 
particular shells—precisely the same difficulty which obtains 
with the operculated Gastropoda. 
When I purposed to offer some remarks on the opercula of 
certain species of fossil shells, I had under my eye four speci- 
mens which, a few years ago, came to my hand in the Wenlock 
Limestone beds on the Longhope side of May Hill. It is true 
that the first examples found explained themselves: they were, 
without a shade of doubt, the opercula of some marine Gastropod, 
but the question as to what species or even genus they could 
belong seemed a hopeless one to settle. The doubt was cleared 
up in a way unlooked for, and that speedily, by my happening 
* Gray’s Systematic Arrangement of the Mollusca in the British Museum. 
London, 1857. Part I., page 67. 
+ ‘“‘Nidamentaldriusendecke”’ is the word forged by our Teuton kinsmen, 
meaning literally ‘‘ nidamentalglandcover.” 
