280 FAMILY NUMMULINIDA. 



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 [ii', it'}. 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 trans- 

 versely between the septa {gy ■, <j(i')- 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 tlic depressions of P. criqnt (although 

 much less pronounced), a minute punctation may be brought into view by careful examina- 

 tion, 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 tlie chambers is thickened b)' an exogenous deposit upon its surface, continuous with 

 tliat of which the central nucleus is composed (f[ 476). 



474. In stud3'ing the internal structure of P. craticidata , I have not been dependent only 

 upon the information afforded by sections of the sliell ; for I have had the opportunity, 

 through the kindness of Messrs. Parker -and Rupert Jones, of examining the beautiful siliceous 

 "casts" which they have obtained by treating with dilute acid specimens whose cavities had 

 been filled by an infiltration of silicate of iron. These casts (of one of which a somewhat diagram- 

 matized view is given in Plate XVI, fig. 9) represent, with the utmost fidelity, the forms and con- 

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

 the shell in the living state of this organism ; and for the reason already mentioned (p. 10) they 

 really afford us more information on those points than we could obtain from the decalcified body 

 the animal itself. In the general shape and proportions of its segments, P. cratlcuJata differs 

 remarkably from P. crispa as from most other nautiloid Foraminifera ; the breadth of each of 

 the later whorls being many times exceeded by the distance between its two lateral surfaces. 

 Thus the segments come to have somewhat of the form and arrangement which the carpels 

 of an orange would exhibit, if, instead of lying in a single circle round a central axis, tliey 

 w^ere disposed in a succession of whorls, with a progressive increase in their dimensions. 

 This comparison may be conveniently carried a little further. For as each carpel of the 

 orange has its own investing membrane, so that the partitions between the adjacent carpels 

 are double, so each segment of PoI^stomeJIa has its own proper shelly investment ; and further, 

 as the separate carpels of the orange are collectively invested by a general integument, which 

 also to a certain degree dips down between them, and which fills up what would otherwise 

 be void spaces about the two poles of the spheroid, so are the proper walls of the spirally 



* In order to distinguish the orifices of these punctations, it is advantageous to remove from tlic 

 surface of the shell that opacity vrhich 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 hvdrocliloric acid as only to e.xert a very 

 feeble degree of solvent power. 



