GENTS OPEECULINA : EXTEENAL CHAEACTERS. 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 by Mr. CUMING 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 WILLIAMSON and Mr. CARTER, it is 

 desirable 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 will 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. CUMLNG'S specimens accord in 

 their general aspect with the one represented in Plate III. fig. 7, which is closely con- 

 formable to Mr. CARTER'S type of Operculina Arabica, 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 flat, 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 spire ; 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 which 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 intervening 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 veiy minute punctations ; but more commonly they are 

 larger and fewer 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 surface 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 



