290 Mr. Toulmin Smith on the Classification 
This species differs very widely in outward appearance from 
C. capitatus. In that species the head is indeed so largely de- 
veloped as to be the most conspicuous part of the entire body ; 
but it still leaves a view to some extent of the wall. In the pre- 
sent species the whole fossil, unless actually looked at from be- 
low, is so entirely enveloped by the cephalic membrane, — rendered 
necessary on account of the great depth and consequent tendency 
to looseness of the fold, — that no idea of the character of the 
membrane of the wall itself can be gained externally. 
The modification of the fold is exceedingly remarkable, and 
exceedingly difficult to be ascertained. The description given, 
however extraordinary it may appear, is the result of very laborious 
and careful examination, comparison, and section of all the spe- 
cimens which I have been able to obtain. A familiar illustration 
may perhaps assist in understanding the arrangement of this 
membrane. If the inquirer will glance at the hangings of any 
window, looped up, as usual, in festoons at some distance from 
the ground by curtain pins or ropes, he will see a contrivance 
rudely imitating the very elegant plan adopted by nature, to give, 
in a small space, a very great extent of surface combined with 
security to the polypiferous membrane of C. campanulatus. Take 
a piece of linen cloth : join together the side-edges along their 
whole length, gathering the lower edge to a point : ffx the upper 
end of the sac thus formed* to a circular plain wire : at a third of 
the length from the bottom fix another wire, which, though alto- 
gether uniting in a circle, is deeply zigzaged : the upper wire 
remaining fixed, raise the lower wire equally all round, and so 
that the drapery hanging from the upper simply circular ware 
shall fall within, and that hanging from the zigzaged circle 
shall fall on the outside. Over both wires draw, smoothly, a 
separate cloth, to which fix both wires. Then, by holding 
the entire contrivance at any point of the plain wire circle, 
the whole will be retained in its place. Such a contrivance will 
afford the best idea of the very remarkable arrangement of the 
internal membrane of the present species, and of the object and 
importance of its deeply extended head. It is obvious that, if 
the lower wire were zigzaged, not only in its horizontal plane but 
also in a direction perpendicular to that plane, though it w^ould 
affect the points at which the outer covering or envelope w^ould 
be touched, it would in no wise affect the principle of the plaits 
or folds whose extremities touched that envelope. The following 
two figures may render this matter still clearer. 
* To act properly, and to give a full idea of the extent of surface gained, 
the sac should be very much wdder at the middle than at the top or bottom, 
in order to fill the lower zigzaged wire and yet inclose the plain fold of the 
cloth without compressing or touching it. 
