19(5 DR. CARPENTER'S RESEARCHES ON THE FORAMIN1FERA. 



distinctly indicated by the surface- mark ings. The only external orifices which 

 communicate with these cells, are the minute pores (d, d, d) forming one or more 

 ranges at the margin of the disk, each pore lying in a vertical furrow, between 

 the projecting walls of two contiguous cells. In the smallest and thinnest disks 

 (Plate V. fig. 1) we find but a single row, or sometimes two rows, of such pores; 

 in disks somewhat thicker, there are three or four rows; and in the largest and 

 thickest Orbitolites (Plate V. fig. 6), no fewer than ten or twelve such rows. This 

 multiplication in the number of ranges of marginal orifices, indicates a like mul- 

 tiplication in the number of floors (so to speak), of which the disk is composed ; 

 and just as the total number of chambers in a building may be increased, either 

 by extending its base over a larger area, or by additions to its number of storeys, 

 so may an increase in the number of segments of which this animal is composed 

 be provided for, either by the marginal addition of a new zone resembling the last, 

 so that the diameter of the disk is alone augmented, or by an increase in the thick- 

 ness of the newly-forming zone, so that it contains a larger number of superposed 

 layers. The new zones, however, never invest or cover those which they surround, 

 each being simply a continuation of the margin of the preceding; and in this respect 

 the mode of growth of Orbitolites at every stage is pointedly distinguished from the 

 early mode of growth of Orbiculina, just now specified as the cause of the protube- 

 rance of its centre. 



14. The shelly substance of the calcareous disk, although firm, is by no means so 

 dense and bony as that of the shells of many other Foraminifera of higher organiza- 

 tion. It is apparently quite homogeneous, rarely presenting the least appearance of 

 'structure,' and this being probably fallacious; I refer to the punctated marking 

 sometimes seen on the outer surface of the nucleus, which shows itself under the 

 aspect represented in Plate VI. fig. 5, when the thin layer of shell which presents it 

 is viewed by transmitted light. Although this appearance might be considered to 

 indicate the existence of a cellular structure in the shell, yet I believe that such an 

 inference would be fallacious ; since I have not been able to detect the least trace of 

 such a structure in the decalcified residuum, which, on the other hand, seems to me 

 to be a substance as structureless as sarcode itself. Coupling these appearances with 

 those which I have found to exist more distinctly in Orbiculina, I am disposed to 

 interpret them as proceeding from minute depressions on the surface; and these are 

 perhaps to be regarded as .the rudiments of those minute closely-set apertures, which, 

 in many Foraminifera, give passage to pseudopodial extensions of the sarcode from 

 every part of their bodies. 



15. In all the forms of Orbitolite that I have examined, the central Nucleus pre- 

 sents the same essential characters. When the interior of any disk, whether large 

 or small, is laid open by a horizontal seotion passing through the central plane, the 

 nucleus is seen to be occupied by a large cavity (Plate V. fig. 1) somewhat irregu- 

 larly divided by a sinuous partition, which always, however, marks out a central 



