260 



FAMILY NUMMULIINIDA. 



pretty obviously communicate with the plexus of passages which it contains ; and thus the 

 interseptal system of one whorl is brought into direct connexion with that of the preceding. 

 It is to be remembered, however, that independently of such connexion, the spiral mode of 

 growth of itself brings about a continuity of the canal-system throughout, by means of the 

 marginal plexus ; the consecutive whorls not being added one to another like the successive 

 annuh of <75^c7oc/y^««, each of which is (so to speak) closed ov complete in itself; but being 

 formed by the prolongation of the spiral lamina and of the marginal cord, which may be 

 considered as always open to indefinite extension. 



Fig. XLIV. 



447. Reverting, now, to the generic differentiation between OpercuUna and Nummulina 

 that has been based by MM. D'Archiac and Haime on the closing-in of the spire, which 

 thev affirm to be a constant character of the latter, whilst in the former the spire seems ordi- 

 narily to open out so long as it continues to increase, — I have to remark that this is by no 

 means an invariable difference ; for I have met with several specimens of OpercuUna 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 growtli. The closing septum of one of these specimens is shown in 

 fig. XLIV. Looking, however, to the circumstance that these specimens 

 bear but a very small proportion to those which exliibit 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 ilis- 

 posed to regard it as an abnormal, or at least as only an occasional 

 occurrence in OpercuUna ; 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 a uniform characteristic of NummuUHa 

 (^ 456). At any rate, the occasional occurrence of this condition in 

 OpercuUna deprives the character of that constancy which is requisite to 

 make it good for generic differentiation ; and if OpercuUna and Nummulina 

 are to be retained as separate genera, I cannot perceive 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 most obvious though not very important 

 characteristics of the former. 



448. It only now remains to speak of certain appearances indicative 

 of reparation after injuries, which throw some light upon the physiology 

 of liiis type of organization. Specimens are not unfrequently met with, 

 exhibiting marked irregularities that may fairly be regarded as having 

 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 of the injury, 

 in a narrowing of that part of the convolution which succeeds it, the 

 new chambers being formed (as it were) on the contracted basis to which their pre- 

 decessors have been reduced. A very remarkable feature in all these reparations is the 



Front view of tlie 

 septal plane closing-in 

 the last chamber of 

 Opercnliiia. 



