GENrS OPEECULINA: — EXTEENAE CHAEACTEES. 
15 
to one particular form of this protean type ; and neither seems in the least degree aware 
of its extraordinary tendency to variation. The rich store of material placed at my 
disposal hy Mr. Cumixg has not only enabled me to prosecute my inquiries into the 
minute structure of this organism upon specimens of unparalleled size and degree of deve- 
lopment, but has also enabled me to bring together a great body of data for comparison 
as to the extent of variation which it may undergo, alike in external conformation and 
in internal organization. And as the information hence obtained has a most important 
bearing upon the study of the closely-allied genus Nummulites^ I venture to think that, 
notwithstanding all which has been done by Professor W illiamson and Mr. Caetee, it is 
desu'able that I should give a detailed account of my own investigations into the 
structure of Operculina, as well as a summary of the results of my comparison of its 
multiform varieties. It udll be advantageous in the first place to examine into what 
appears to be the characteristic structure of the type, and then to inquire into the 
degree of modification to which this may be subject on each point. 
142. External characters . — A large proportion of Mr. Cuming’s specimens accord in 
their general aspect with the one represented in Plate III. fig. 7, which is closely con- 
formable to Mr. C.urtee’s type of Operculina AraUca, except in its somewhat larger 
dimensions. It is a compressed spiral of about '25 inch in diameter, and about '015 inch 
in thickness, consisting of between three and four convolutions gradually increasing in 
breadth ; these are in general nearly fiat, but are sometimes a little arched between their 
inner and outer margins, sometimes depressed so as to present a slight concavity, espe- 
cially near the outer margin of the last whorl. The chambers are about seventy-five in 
number, commencing from a primordial spheroidal cell and progressively increasing in 
dimensions with the widening of the sphe ; their septa have for the most part a radial 
direction, but they bend backwards near the outer margin of each whorl ; and they are 
marked externally by bands wdiich are distinguished by their semitransparent aspect 
from the dull brownish hue of the general surface. These septal bands are commonly 
on the same plane with the inteiwening portions of the shell ; but sometimes the walls 
of the chambers are a little arched between the septa that bound them, so that the 
septal bands are slightly furrowed ; whilst the walls of the chambers are sometimes a 
little depressed, so that the septal bands are prominent ; and such varieties may present 
themselves in different parts of one and the same shell. Not unfrequently the whole 
surface is seen to be marked by very minute punctations ; but more commonly they are 
larger and few^er in number, and are often arranged in pretty regular lines parallel 
to the septal bands, — one, two, or three rows of such punctations being seen on the wall 
of each chamber (Plate V. fig. 7) ; these punctations, when sufficiently magnified, are 
found to be spots of semitransparent shell-substance resembling that of the septal 
bands ; and, as in the case of these, their smfface is sometimes on the same plane with 
that of the general surface of the shell, sometimes a little elevated so as to form papillae, 
and sometimes a little depressed into minute fossae (fig. 8). These punctations often 
present themselves abundantly on some parts of the surface, whilst they are entirely 
absent from others. Sometimes, instead of a limited number of comparatively large and 
