GENUS ORBITOLITES. 119 



margin, which, nevertheless, they completely surround. A careful examination oi this 

 specimen, indeed, seems to me to leave little room for doubt, that the growth of the inner- 

 most or what I may call the rcparatirr zone of chamberlets took place, not from the broken 

 edge, but from the margin of the unbroken ; just as, to use a professional simile, an ulcerated 

 surface " skins-over" by an extension of the integument from its edges, not by the direct 

 formation of skin upon the granulation-surface itself All the six rows subsequently pro- 

 duced are conformable to each other and to the first or reparative row, from which they 

 have obviously extended themselves after the normal manner. It is observable, however, 

 that the breadth of these rows varies in different parts, being least where they invest the 

 projecting portions of the fractured edge, and greatest where they sink into its hollows. 

 And thus it comes to pass that the irregularities left in the shape of the disk, by the loss of 

 a large part of its substance, are gradually compensated, so as to restore it to a form much 

 more nearly corresponding to its typical symmetry. It is interesting to find evidence in 

 fossil specimens, that the same kind of reparation has taken place. Among the Orbitolites 

 which I have examined from the Calcaire grassier of Paris, is a disk of which a large part had 

 obviously been lost by fracture, but of which the original symmetry had been in great degree 

 restored by a similar outgrowth from the zones formed from the uninjured margin, along 

 the fractured edge. — In the specimen represented in fig. 27, in which but a very small frag- 

 ment appears to have served as the nucleus for a new disk, the tendency to the reproduction 

 of the typical form, by the compensative reparation just described, is still more curiously 

 marked. This specimen also presents the very unusual feature, that the new growth has 

 taken place from the inner margin of the original fragment {a, a), and not from its outer or 

 growing margin, as in the case previously noticed. Having carefully examined it in various 

 modes, I cannot entertain the slightest doubt that such has been the case ; for the chamber- 

 lets of the first new zone, as well as those of all the zones subsequently produced, are so 

 manifestly conformable to those of the thinner and older portion of the fragment, and are so 

 unconformable to those of the thicker and newer margin, that it seems obvious that the 

 sarcode must have extended itself from the former part, along the fractured edge on each 

 side, and have then enveloped the margin which had been left entire. This may have more 

 readily taken place in the present instance, because at the part a, a, the fracture seems to 

 have followed the course of one of the zones, instead of passing (as at the sides of this 

 fragment, and as in the instance previously cited) in such a direction as to cut the zones trans- 

 versely. — Again, I have met with several specimens, in which the central portion of the disk 

 having been broken-out, a growth of new zones seems to have taken place from without 

 inwards, so as to fill up the void space thus left ; the included portion being evidently as 

 unconformable to that which surrounds it as in the preceding case, and a void remaining 

 unfilled, the shape of one part of which clearly indicates that it occupies the site of the 

 original centre ; — so as to render the conclusion almost inevitable that the included portion, 

 and not the peripheral, must be the after-growth. 



176. This series of abnormal phenomena, then, not only confirms the conclusion that 

 seemed fairly deducible from our previous examination of the normal mode of growth, with 

 regard to the independent endowments of the component segments of the sarcode-body of 

 Orbitolites, but also affords some additional information of much interest. For we see, in 



