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 : fix 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 wire 

 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 would 

 affect the points at which the outer covering or envelope would 

 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 wider at the middle than at tlie 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. 



