GENUS OPERCULINA. 201 



continuation of llic marginal cord along the fractured edge. It might have been supposed 

 tliat the wound would have been healed with some rude exudation, which would have 

 given rise to an amorphous shell-structure; but instead of this, we find the reparation 

 chiefly effected by a new production (probably by extension from the old) of that portion 

 which gives the most evidence in its canalicular striicture of being intimately connected 

 with the vital changes taking place in the organism. In one specimen in my possession, 

 a reversal seems to have taken place in the direction of growth after the fracture, the 

 spire having been broken across, almost through its centre, after the first two convolutions had 

 been formed; and the extension of the marginal cord which has closed-in the fractured por- 

 tion having proceeded not from the later but from the earlier of the two fractured extremi- 

 ties of the second convolution, its backward growth produces a reversal in the direction of 

 the spire. A somewhat similar retrograde increase has been alrcad)^ noticed among the 

 modes in which Orbitolifes is occasionally repaired \*\ 17.5) ; and it obviously shows that 

 every portion of the organism is equally capable of extending itself when left free to do so. 

 It is worthy of remark that the growth of this specimen subsequently to the injury has 

 taken place more after the plan of NuDuiuilinn than on the ordinary plan of Opercidina, the 

 convolutions being more numerous than usual and their rate of increase slow : there is 

 evidence, however, in the presence of a small fragment of the final whorl, that it underwent 

 the thinning-out which is characteristic of its type. — Notwithstanding the great number of 

 specimens which I have examined, I have not met with one that presented any such depar- 

 ture from the normal type of growth as woidd deserve to be termed a monstrosity ; so that 

 it would seem as if such aberrations were more frequent in the cyclical than in the helical 

 forms of Foraminifera. 



449. Afjinities. — Since in this genus we have a complete development of all the cha- 

 racters that essentially distinguish the Nummuline type, it is obvious that it can bear no 

 direct relationship to any true Rotahne ; and although several approximations to the form of 

 Operculina are presented in the Rotaline series (^^ 353, 359, 367), yet as these are deficient 

 in all its other distinctive features, its relationship to them is one of analogy only, not of true 

 affinity. To Jinjjhistcgina it is obviously related much more closely, but it is difi^erentiated from 

 that type not only by its perfect symmetry and by the suppression of the alar and astral lobes, 

 but also by the development of the additional septal lamella, of the marginal cord, and of the 

 canal system. With Nummnlina the relationship of Operculina is so intimate, that, as we have 

 seen, it is difficult to find a valid generic distinction between them ; that suppression of the 

 alar lobes which constitutes the most obvious differentiation between the typical Operculina 

 and the typical Nummulince, being a character which (as we shall presently see, ^ 462) presents 

 itself in the " assiline" group of true Nummulites. Although there is no very intimate rela- 

 tionship between the typical Operculina and the typical Polystomella, yet the approximation 

 between some of the least developed examples of the former and the " nonionine'' forms of the 

 latter is so close that it is difficult to distinguish them ; and the essential distinction will 

 probably be found to lie in the distribution of the canal-system respectively characteristic of 

 these two types. To lleterostcgina the relationship of Operculina is extremely intimate ; 

 nothing more being required than the subdivision of its chambers into chamberlets, to con- 

 vert the latter into the former. Such a subdivision we have seen to occur in numerous other 



