GENUS OPEECULINA: INTEENAL STETJCTUEE. 



29 



Fig. IX. 



162. Hence it would appear that there is no other essential difference between Oper-. 

 culina and Nummulites *, than that which consists in the closing in of the spire, 

 affirmed by MM. D'ARCHIAC 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'ARCHIAC 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 spire, 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 ai - e deli- 

 neated in fig. 1, Plate V. ; and we can scarcely, I think, be wrong 



in concluding that these irregularities have commonly been produced by fracture. A 

 portion of the outer part of the spire 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 



Front view of the septal 

 plane closing in the last 

 chamber of Operculina. 



in his Fourth Plate, of the chambers and passages in different species of an organism which he designates 

 Orbitoides, being in the closest conformity with my representations of the corresponding parts of these two 

 genera, between which the form described by Professor EnEENBEBa seems to be a connecting link. 



* I have already (^f 104, note, Philosophical Transactions, 1856, p. 558) corrected the mistake into which 

 I fell, when treating of the structure of Nvmmulites, in regarding the non-tubular columns of the fossil shell 

 as having been passages subsequently filled up. 



