PRINCIPLES OF CLASSIFICATION. 53 



FoRAMiNiFERA ; and this primary differentiation will be found so constantly to harmonize 

 with the grouping which would be based on the principle of continuity of gradation, that I 

 cannot entertain a doubt of its being the one on which (in our present state of ignorance 

 respecting the physiology of this tribe) our classification may be most securely based. For it 

 will be found that the several types of " porcellanous" Foraminifera, however diversified in 

 form, constitute a series connected throughout by the closest links of mutual affinity ; whilst 

 the proper " arenaceous" types form a parallel series, of which the members are not less 

 closely related to each other. In like manner we shall find that the far more numerous 

 forms of the " hyaline" Foraminifera may be ranked together into a small number of assem- 

 blages, the members of each of which are so gradationally united that the groups thus composed 

 are obviously in the highest degree natural ; whilst these series are at the same time connected 

 with each other, not only by their mutual approximation to certain fundamental types, but also 

 by intermediate forms which serve to link together even their more divergent portions, the 

 whole being thus united into a compact aggregate, of which there is no part that is not held 

 {as it were) to the rest by firmly cohesive attraction. — -It must not, however, be supposed that 

 every member of the one series is differentiated by the characters just enumerated from everv 

 member of the other. The non-perforation of the walls of the chambers, the singleness of 

 the septal partitions, and the freedom of the apertural communications between the chambers, 

 seem invariably to characterize the shells of the porcellanous type ; whilst the perforation of 

 the walls of the chambers, the duplication of the septal partitions, and the straitness of the 

 apertural communications, appear to be no less constant characteristics of the shells of the 

 hyaline type. The "intermediate" or "supplemental" skeleton, on the other hand, presents 

 itself to an extent that makes it readily distinguishable, only in the most developed forms of 

 the hyaline series ; and it is not a little remarkable that within the limits of one and the same 

 generic type {T/iio/joi7/s) we meet with certain forms in which this portion of the fabric is evolved 

 to an extraordinar)^ degree, whilst in others its presence is not to be traced at all. And the 

 same may be said of the " canal-system," which presents its most extensive and symmetrical 

 distribution in the highest examples of certain generic types, whose less elevated specimens show 

 scarcely any traces of it. Hence, whilst the presence of an "intermediate skeleton'' and of a 

 " canal-system" in one Foraminiferous shell serves to mark it as belonging to the higher section of 

 the hyaline series, their absence in another must not be regarded as indicating its porcellanous 

 characters, seeing that such absence prevails equally through the lower section of the hyaline 

 series also. And it is chiefly because their presence, in connection with other characters, serves 

 to mark a /iis/f>i or tendency in the one series, of which no trace whatever is presented by the 

 other, that it has been here included among the features of distinction between the two. 



66. On the other hand, there is an entire absence of any other special relation between 

 the members of the " porcellanous " and those of the " hyaline " series, than that which 

 arises out of the configuration of their respective shells ; which configuration is determined b_v 

 their plans of (jrowtli, that is, by the direction in which new chambers are successively added. 

 Thus, in both series we have rectihneal, spiral, cyclical, and acervuline forms, with everv 

 gradation between these ; and it is true that such a striking isouiorpliisiii. displays itself between 

 certain types of these two series respectively, as would not unnaturally lead any svstematist 

 whose views of classification of Foraminifera might be founded on their supposed Molluscous 



