540 DE. CAEPENTEE'S EESEAECHES ON THE FOEAMINIFEEA. 



constitute the only interruption to the general uniformity of the surface, the septal 

 bands not being clearly distinguishable ; and this disposition is commonly found to 

 prevail on the surface of the inner whorls, when it is exposed by the removal of the 

 outer (M', if). In the newer portion of the last formed whorl, on the other hand, we 

 may observe that instead of each punctation having a separate dimple of its own, the 

 corresponding punctations of the two rows lie in a succession of furrows that pass 

 transversely between the septa (gg', gg'). In the most recently formed portions of 

 specimens that have not attained their full growth, we find these furrows to be deeper 

 towards the posterior than towards the anterior margin of each interseptal space ; and 

 in the deepest portion of each of these furrows, which obviously correspond with the 

 "fossettes" of P. crispa (although much less pronounced), a minute punctation may be 

 brought into view by careful examination, a corresponding row of punctations being 

 also traceable on the other side of the septum*. These varieties of superficial aspect 

 may present themselves on different parts of one and the same specimen ; and it will 

 appear from the explanations which I shall presently have to furnish, that they are 

 occasioned by differences in the degree in which the proper external wall of the 

 chambers is thickened by an exogenous deposit upon its surface, continuous with that 

 of which the central nucleus is composed (^f 185). 



181. Internal Structure. By the fortunate contingency already adverted to, I have 

 been enabled to study the internal structure of this remarkable type, not merely by 

 examining thin sections of the shell taken in various directions, and by comparing the 

 appearances they present with those obtained by laying open its interior by fracture ; 

 but also by submitting to microscopic examination siliceous " casts " of its cavities and 

 channels, which appear to represent with the utmost fidelity the forms and connexions 

 of the various parts of the sarcode-body which occupied those cavities and channels in 

 the living state of this organism. Between the results of these two modes of study I 

 have found the conformity to be so exact, that the account of the structure of the 

 animal which I should have given from examination of the shell alone, has not needed 

 to be modified in the slightest particular by the information more recently furnished by 

 these " casts ; " and, in point of fact, the ideal representation in Plate XVIII. fig. 1, which 

 was entirely based on the former source of information, has not needed the least altera- 

 tion to bring it into accordance with the exact delineations of the latter class of objects 

 which are given in figs. 12, 13f. 



182. In the general shape and proportions of its segments, this type of Poiystomella 



* In order to distinguish the orifices of these pimctations, it is advantageous to remove from the surface 

 of the shell that opacity which it derives from abrasion, and to get rid of the fine particles of calcareous 

 matter which often choke up and obscure its pores. This is readily effected by immersing it for a short 

 time in water so slightly acidulated with nitric or hydrochloric acid as only to exert a very feeble degree of 

 solvent power. 



f I think it well to state this circumstance, as it may increase the confidence accorded to my descriptions 

 of other types, of which the shells alone have been submitted to examination. 



