58 



OF THE FORAMINIFERA GENERALLY : 



septal plane in different individuals can be greater than that which often presents itself at 

 different periods of the life of one and the same individual. Thus, among tlie nauiiloid 

 Foraminifera, there is very commonly to be noticed a remarkable tendency to flattening- out 

 in the latest whorl ; the breadth of the spire being rapidly augmented, whilst its thickness 

 (or the space between its two lateral surfaces) is proportionally diminished. Of this, again, 

 we find a striking illustration in Operculina ; in which the attenuation of the last convolution 



Fig. X 



^. 



Vertical section of the tluee outer convolutions of an Opcraihna, sliowing a complete change in the form and proportions 



of the last convolution. 



Fig. XI. 



often proceeds to a far greater extent than in the example represented in the accompanying 

 fio-ure, the septal plane being thus converted in one turn of the spire from the form of a broad 

 arrow-head to that of a band so narrow as to be little else than a line, whilst it is 

 leno-thened in the same proportion. The form of the septal plane, moreover, is in direct 

 relation, not merely with the general contour of the shell, alike in its lateral and in its antero- 

 posterior aspects, but also with certain of its surface-markings. For if each chamber be 

 merely applied to the extremity of that from which it is budded forth, the whole of the 

 previously formed shell remains visible externally ; and thus, in the case of a nautiloid spiral, 

 all the whorls are traceable from its commencement to its termination. But it most commonly 

 happens that the earlier whorls are either partially or completely invested by the later: 



where such investment is complete, nothing but the last whorl is 

 visible externally (Fig. XI) ; where, on the other hand, it is par- 

 tial, the earlier whorls maybe more or less clearly distinguished. 

 Now, the degi-ee of this investment is determined by the degree in 

 which the successive segments of the sarcode-body of the animal 

 send out lateral lobes that extend themselves over the previously 

 formed portion of the shell ; and this is manifested in the shell by 

 the development of what may be termed the alar prolongations 

 of the chambers, which are the portions formed to include 

 those lobes. Thus, in Fig. XII, we have an anterior view of 

 a nautiloid shell, in which the last whorl so little encroaches 

 on the preceding, that the septal plane s.p. terminates at the 

 an"-les h, h, which are but little nearer to the centre of the spire than the aperture a, which 

 lies against the margin of the preceding convolution. On the other hand, in Fig. XIII, we have 

 a like view of another nautiloid shell, in which the last whorl completely invests the preceding, 



Lateral view of a nautiloid shell, 

 of which each convolution com- 

 pletely invests the preceding, 

 so that the chambers 1-S are 

 hidden bv 9-15. 



