30 DR. CARPENTER'S RESEARCHES ON THE FORAMINIFERA. 



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 predecessors have 

 been reduced. A very remarkable feature in all these reparations is the continuation of 

 the marginal cord along the fractured edge. It might have been supposed that 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 structure of being intimately connected with the vital 

 changes taking place in the organism. This phenomenon is beautifully displayed in the 

 specimen represented in fig. 4, Plate VI., which exhibits another remarkable feature, for 

 which it is difficult to account, a reversal in the direction of growth which has taken 

 place after the fracture. Although the specimen is unfortunately not complete, there 

 can be (I think) no reasonable doubt that its convolutions must have taken the course 

 indicated by the dotted lines ; and it would seem that after the first two convolutions 

 had been formed, the spire had been broken across, almost through its centre. It will 

 be at once seen, by comparing the direction of the septa of the second convolution with 

 that of all those which surround it, that the latter have been reversed ; and although the 

 part in which the reversal actually took place is unfortunately wanting, yet there can be 

 little doubt that it was made as indicated in the figure (a). But the most curious feature 

 in the specimen is this, that the extension (ac) of the marginal cord which has closed in 

 the fractured portion has obviously proceeded not from the later (), but from the earlier 

 (c) of the two fractured extremities of the second convolution ; and it is in this back- 

 ward growth that the reversal would appear to have originated. A somewhat similar 

 retrograde increase has been already noticed among the modes in which Orbitolites is 

 occasionally repaired (*J[ 39) ; 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 Nummulites than on the ordinary plan of Operculina; 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 departure 

 from the normal type of growth as would 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 

 type. 



Genus AMPHISTEGINA. 



164. History. This genus was first constituted by M. D'OKBIGNY in 1825, for the 

 reception of a type of Foraminiferous shells which does not seem to have been previously 

 noticed by those who have given their attention to this group, in consequence, it may 

 be, of its limitation to the seas of warm latitudes. Notwithstanding the very close rela- 

 tionship which, as I shall presently show, it bears to Nummulites and other Nautiloid 



