GENUS ORBITOLITES. 109 



seen to be much higher than they are broad, so that they present a somewhat coUimnar 

 form ; the proportion of their height to their breadth, however, may vary greatly in different 

 parts of the same disk, the former often increasing from the centre towards the periphery, 

 whilst the latter remains constant, or nearly so; and the columns, instead of being straight, 

 are generally more or less curved, and are sometimes bent in the middle at an obtuse angle. 

 The gradation which presents itself from one of these forms to the other, and their coexist- 

 ence even in the same specimens, clearly prove that no value can be attached to the form 

 and proportions of the chamberlets, thus seen in a vertical section, as furnishing specific 

 characters. In every perfect specimen the columnar chamberlets are seen to be closed at 

 their two extremities by a thin wall of shell ; and this is sometimes flat, sometimes more or 

 less convex. 



158. In this manner any number of concentric zones may be formed, which are exact 

 repetitions of each other, except that the number of chamberlets in the outer zones is greater 

 than that of which the inner zones are composed. It does not increase, however, in the 

 regular ratio of the respective diameters of the zones ; for the chamberlets of the outer 

 zones, being usually both larger and more widely separated from each other than are those of 

 the inner, are less numerous in proportion ; thus, in a specimen before me, there are twenty- 

 eight in the innermost row and only forty-nine in the outermost, though the latter is more 

 than twice the diameter of the former. The augmentation in number is accomplished by 

 the occasional interpolaiion of an additional chamberlet, communicating directly with the 

 one immediately interior to it, between the two chamberlets which are connected with the 

 annular passage on either side of the latter, as is shown in Plate IX, fig. 6, c' , c". The 

 chamberlets of the last-formed zone communicate with the exterior by the same kind of 

 radial passages as in other instances communicate with the next zone ; and the external 

 orifices of these form the pores which present themselves at the margin of the disk (Plate IX, 

 figs. 1, 2,/,/). Thus it is seen, on the one hand, how it happens that these pores are 

 intermediate between the chamberlets, instead of opening directly into them ; and on the 

 other, how each pore leads, by the divarication of its passage, into two chamberlets, one on 

 either side of it. When a new zone is formed, each pore opens into one of its chamberlets ; 

 and this zone, in its turn, communicates with the exterior through a new set of pores at its 

 own margin. When the section passes through the prominent annulus of shell which 

 surrounds each pore, this will be indicated by a little " beak" on either side of the entrance 

 to the passage ; such " beaks" (which are, of course, repeated through the entire disk) 

 are shown in their ordinary aspect in Plate IX, fig. 6, /, /), but they are frequently 

 more prominent. In all cases in which the growth of the disk takes place with 

 normal regularity, a complete circular zone is added at once. Exceptions to this regu- 

 larity are rare, and they can be generally traced with probability to some accidental 

 interruption. 



1 59. It is a fact of much importance, in the due appreciation of the relations of 

 Orhilolifes to other types of Foraminifera, that the calcareous partition which separates each 

 chamberlet of any one zone from the adjacent chamberlets on either side is not double, but 

 siyic/le. And this is in great part the case, even with regard to the partitions which separate 



