574 
DE. CAEPENTEE’S EESEAECHES 0 ^ THE FOEA^^imFEEA. 
transitional conditions of aperture, are so obviously related to both the foregoing, that 
no reasonable doubt can exist as to their derivation from them. Now the geographical 
distribution of the two fundamental types is so far different, that where one prevails the 
other is either absent altogether, or presents itself under a modified form ; and thus we 
seem justified in the belief that, whether either of them has been derived from the other, 
or both have been derived from some intermediate form (such as that which seems 
common alike to the young of both), the modifications which have given rise to the 
marked differences they now exhibit are mainly due to diversities in the external con- 
ditions under which they have been respectively propagated. 
244. But to what other type does PencropUs itself present the closest approximation 1 
By systematists in general the intimate relationship which I have shown it to possess to 
the helical type of OrbicuUna 130) has been so slightly regarded, that it has been 
considered as at least equally related to the Operculine type ; and yet, as I shall pre- 
sently show, these two types are removed from each other in all the most essential 
features of their structure, as far as any two polythalamous Foraminifera can be. And 
the idea of the derivation of Peneroplis from the same stock ufith OrbicuUna, seems 
justified by the fact that the young forms of the two are fr-equently so alike as not to 
be distinguishable by external characters alone, whilst tlieii- internal difference consists 
only in the presence or absence of the secondary or transverse septa, a character which 
I have shown reason to regard as variable in this group* 130). 
245. Nofrvithstanding, therefore, the apparently ■v\ide divergence of the cychcal Orbi~ 
tolites, the helical OrbicuUna, the fusiform Aheolina, and the simply-chambered Penc- 
Toplis and Pendritina, these several types must be regarded as most intimately related 
to one another; and that relationship seems to me much more likely to have arisen 
from a common ancestral descent, than from the original creation of independent types 
capable of graduating into each other so continuously as almost to assume each other's 
characters. It is very important to remark that they all possess that peculiar textiu'e of 
shell, which is designated by Professor Williamson as ; presenting an opake 
white hue when seen by reflected light, but a rich brown or amber colour when seen by 
fight transmitted through thin natural lamellae or artificial sections. This substance is 
entirely structureless, and possesses no great density or tenacity. Moreover in all the 
foregoing types, each of the septa intervening between the chambers consists of only 
a single layer ; the passages of communication between them are for the most part so 
large and free, that the segments of the sarcode-body are but very imperfectly isolated 
from each other ; and, as might be anticipated from this incompleteness of separation, it 
is here that variations in the mode of communication between the chambers seem to be of 
* My statement on tliis point is fully confirmed by Messrs. Pakkek and EurEBT Joxes ; wbo state that 
“not unfrequently, feebly-developed peneropliform varieties, as well as good-sized adunciform specimens, 
occur, in which the long narrow chambers are at times simple and imdivided, being occupied by transversely 
elongate lobes of sarcode, instead of numerous minute subcubical blocks.” See Ann. of Hat. Hist., March 
1860, p. 180. 
