126 FAMILY MILIOLIDA. 



of the present time, it is most complete in the Orbitolites of our Southern Ocean. These 

 two types are at once distinguishable from each other, however, by a comparison of the centres 

 of their respective disks ; for not merely does OrhicuUna invariably commence on the spiral 

 plan, but it invariably persists in this until the spire has made three or four turns, of which 

 each invests the preceding, so as to augment the thickness of the centre, and to cause this to 

 project as a rounded knob above the plane of the peripheral portion of the spire or disk. In 

 Orbitolites, on the other hand, the cyclical mode of growth seems characteristic of the type ; 

 for even when the early growth follows a spiral arrangement, this seems simply to result from 

 a defective power of gemmation in the " primitive mass " of sarcode, and the spire never 

 proceeds beyond a single turn, or encroaches on the surface of the primitive disk, but from 

 the iirst shows a tendency to pass into the cyclical form. 



184. The relation of Orbitolites to Tinoporus (the Orbitolina of D'Orbigny), to Orbitoides, 

 and to Cycloclypeus, is one of mere similarity in mode of growth, and consequently of analogy 

 only ; their essential characters being such as to remove them most widely from it. For as 

 Orbitolites is the cyclical type of the Milioline series, so shall we see that Tinoporus stands in 

 the like relation to the Rotaline, and Cycloclypem to the Operculine, whilst Orbiloides seems 

 to be a connecting link between the two latter. 



185. Geographical Distribution. — This type, like OrbicuHna, is pretty generally diffused 

 along the shores of the warmer seas ; but it is interesting to observe that it is most abundant 

 where Orbiculina is comparatively rare, and vice versa. Thus, it is more commonly met with 

 in the Australian and Polynesian seas than in the West Indian or Philippine, and is peculiarly 

 abundant in the Red Sea (the shell-sand of Suez yielding it in extraordinary copiousness), 

 though not there attaining any large size or high development, whilst it seems to die out in 

 the Mediterranean, the specimens gathered on its shores being all of stunted growth. Its 

 largest size and highest development at the present time are attained in the Polynesian seas, 

 especially on the sides of Coral-reefs, where the disks that have become detached from the 

 sea-weeds to which they are usually attached, often accumulate to an enormous extent. 



186. Geological Distribution. — The early part of the Tertiary period appears to have 

 been unusually rich in Foraminifera of the largest size. The C'alcaire Grossier of the Paris 

 basin, and corresponding formations in the south of Europe, contain enormous numbers of 

 Orbitolites, which often attain a diameter of 8- 1 Oths of an inch ; and the Limestones of the north- 

 west of India, which in some parts are rich in Orbicidina and Qrbitoides, are elsewhere almost 

 entirely composed of an equally large variety of Orbitolites, which Mr. Carter (xix) has 

 described under the name of Cycloliua, supposing it (from the peculiarly cyclical aspect of its 

 surface, 1178) to belong to the genus so named by D'Orbigny, which seems, however, to 

 have been really founded on a varietal form of Tinoporus. The true Orbitolites first makes 

 its appearance in the rich Polyzoic deposits of the shallow-water Maestricht chalk ; and it 

 has probably continued to inhabit the ocean waters, from the time when it first accumulated 

 so as to form an important constituent of the Eocene Limestones, down to the present epoch. 

 Whenever the Coral islands at present submerged shall in their turn undergo elevation, a new 

 series of Orbitolite-limestones, now in progress of formation, will probably be brought to light. 



