GENUS OPERCULINA: — INTEENAL STEUCTUEE. 
29 
162. Hence it would appear that there is no other essential difference between Oper- 
culina and Niimmulites* , than that which consists in the closing in of the spire, 
affirmed by MM. d’Aechiac and Haime to be a constant character of the latter, whilst 
in the former the spire seems ordinarily to open out so long as it continues to increase. 
But even this is not an invariable difference ; for I have met with several specimens of 
Operculina, in which the spire closes in by a somewhat abrupt inflexion of the marginal 
cord, so as to produce a rapid diminution in the size of the last four or five chambers, 
ending (as it would appear) in a complete cessation of growth. Examples of this kind 
are represented in fig. 8, Plate III. and fig. 3, Plate V. ; and in Fig. IX. is shown the 
closing septum of one of these specimens upon a more enlarged 
scale. Looking, however, to the circumstance that these specimens 
bear but a very small proportion to those which exhibit no such 
tendency, and also to the fact that some of the specimens in 
which this closing-in is seen are very far from having attained 
their full growth, I am disposed to regard it as an abnormal, or 
at least as only an occasional occurrence in Operculina', and I 
must confess that, notwithstanding the positive assertion of 
MM. d’Aechiac and Haime, I still entertain doubts as to whether 
it is to be accounted as a uniform characteristic of Nummulites. 
At any rate, the occasional occurrence of this condition in Oper- 
culina deprives the character of that constancy which is requisite 
to make it good for generic differentiation ; and if Operculina and 
Nummulites are to be retained as separate genera, I cannot per- 
ceive by what features they are to be distinguished, save by the 
marked compression in form, the limited number of convolutions, 
and the external display of the whole spLe, which are the obvious 
though not very important characteristics of the former. 
163. It only now remains to speak of certain appearances indi- 
cative of reparation after injuries, which throw some light upon 
the physiology of this type of organization. Specimens are not 
unfrequently met with, exhibiting such irregularities as are deli- Eyont view of the septal 
o o plane closmg in the last 
neated in fig. I, Plate V. ; and we can scarcely, I think, be wrong chamber of Operculina. 
in concluding that these iiTegularities have commonly been produced by fracture. A 
portion of the outer part of the spffie being broken off, the wound heals by the formation 
of new shell at the margin ; but in its further progress the spire often shows the effect 
in his Eourth Plate, of the chambers and passages in different species of an organism which he designates 
Orhitoides, being in the closest conformity with my representations of the corresponding parts of these two 
genera, between which the form described by Professor Eheenbeeg seems to be a connecting link. 
* I have already 104, note, Philosophical Transactions, 1856, p. 558) corrected the mistake into which 
I fell, w'hen treating of the structure of Nummulites, in regarding the non-tubular columns of the fossil shell 
as having been passages subsequently filled up. 
