122 FAMILY MILIOLIDA. 



Orbitolites, yet it is by no means restricted to this ; being frequently met with in the thicker 

 disks of the more Complex type, and being almost constant in the fossil forms that abound 

 in the early Tertiaries. Its occurrence, however, may always (I believe) be considered as 

 indicating an incomplete separation between the superficial segments and the columnar seg- 

 ments of the intermediate stratum (f[ 1 73) ; so that the former present the shape of the latter, 

 in place of that which properly characterises them. The shape of the chamberlet is 

 sometimes marked out in unusual strength by the convexity of its roof or cover ; and this 

 feature is often so pronounced in the large fossil Orbitolites of the Paris basin, as to become 

 visible to the naked eye. A veiy marked diversity in its degree, however, as w^ell as in the 

 size of the cells, is often to be noticed in contiguous zones ; whence it is obvious that the 

 convexity is a mere accidental variation, and is a character of no value whatever as regards 

 the differentiation of species. The relation of the rounded to the square or rectangular 

 charnberlets is made evident by the occurrence of intermediate Unks of transition. — The 

 foregoing considerations seem to render it obvious, that diversities in the form of the super- 

 ficial chamberlets do not afford any ground whatever for the estabHshment of a corresponding 

 multiplicity of specific types, but that they must rank as individual variations to which there 

 is scarcely any definite limit. 



179. Besides those regular markings of the surface which correspond to the division 

 of the interior into chamberlets, a peculiar aspect is frequently given to it by extraneous 

 calcareous deposits, which are sometimes irregular, but which occasionally present an 

 approach to radial symmetry. It is worthy of note that these deposits present themselves far 

 more frequently, and also in a far more characteristic manner, in the Orbitolites of the 

 Philippine Seas, than in those of the Australian or of any other provinces ; and this circum- 

 stance seems to render it probable that the outgrowth is directly due to the influence of 

 some external condition, probably an excess in the proportion of carbonate of lime in the 

 w'aters inhabited by these particular specimens. 



180. Although the cyclical Mode of Growth, w-hen once established, is subsequently 

 maintained with great regularity, and although in what may be considered the typical form 

 it commences from the " primitive disk " itself, yet there are numerous instances in which 

 the typical regularity is more or less widely departed from, so that the early increase seems 

 to take place after an altogether different plan. The most marked antithesis to that regu- 

 larly concentric type of growth, in which a complete annulus of chamberlets immediately 

 surrounds the primitive disk (see Fig. XXIV, •[[ 157), is presented by those forms in which 

 the circumambient segment has only given origin to new segments at its extremity ; these 

 in their turn bud forth others, which extend and multiply themselves laterally as well as in 

 advance ; and thus a kind of spiral is produced, which opens out very rapidly, the lateral 

 portions of its mouth tending to grow round and embrace the primitive disk. An example 

 of this kind, in which as many as twenty-two zones (counting the primordial segment as the 

 first) succeed one another before the first complete annulus is formed, is shown in Plate IX, 

 fig. 5. Another example of the like abnormality, taken from a specimen in which the " primi- 

 tive disk" had a remarkably Milioline aspect, is shown in Plate IV, fig. 21. Now if these 

 two plans of growth — the one cj/clical from the beginning, the other cyclical only after having 



