472 ORIGINAL ARTICLES. 



segments much more independent of one another, than they are in the 

 porcellanous type; and their isolation is marked by these two im- 

 portant peculiarities in the structure of the shell, — first, that each 

 segment has its own complete wall, so that the septa between suc- 

 cessive chambers are double, — and second, that the apertures of com- 

 munication through the septa are far smaller than in porcellanous 

 shells, as is seen in comparing a Vertebralina or Miliola with a 

 Nodosaria or Cristellaria, or, in the unilocular types, on comparing 

 the aperture of a Gromia with that of a Lagena. It is in this type 

 alone that we meet with an " intermediate skeleton " nourished by a 

 " canal system " that is connected with the cavities of the chambers ; 

 although this feature is wanting in the lower types of the series, yet 

 its presence in the higher, most strongly differentiates them from the 

 forms of the porcellanous type to which they bear the closest resem- 

 blance. In certain genera of this as of the porcellanous series, we find 

 the surface of the shell occasionally roughened by the adhesion of are- 

 naceous particles; but these are imbedded in true shell- sub stance, 

 which is never wanting ; and as the very same forms may be altogether 

 free from arenaceous deposit, its presence is obviously not essential 

 but is (so to speak) accidental, and constitutes no ground for even 

 specific distinction. 



As the texture of the shell throughout the whole of this series is 

 essentially the same, — the variation in the diameter of its tubuli 

 being the only difference of any mark, — we have not the same easy 

 means of subdividing the Perforated group into families as we possess 

 in the case of the Imperforate ; and this division must consequently 

 be based on the aggregate of characters supplied by the coarseness or 

 firmness of the tubuli, the mode of communication between the cham- 

 bers, and the general plan of growth. To enter into details upon these 

 points would be foreign to my present purpose, which has been 

 merely to set forth the general results at which I have arrived ; and 

 these I now offer to the criticism of such Naturalists as interest 

 themselves in the study of the group to which they relate. 



tubuli ; and it comes to be a very interesting inquiry what relation there may be 

 between these two substances as to the mode of their formation. There is reason 

 to consider the shell-substance of the Foraminifera as an excretion from the proto- 

 plasmic mass of which the body itself is composed ; just as the cellulose wall of the 

 vegetable cell, which may be consolidated by carbonate of lime (as in Corallines) or 

 by silcx (as in Diatoms) is an excretion from the contained endochrome. The 

 new lamella? of shell successively added to the external surface of the preceding, in 

 cases in which the spiral lamina of each new whorl completely invests the old, would 

 block up its pores, if the continuity of the tubuli were not maintained by the exten- 

 sion of the pseudopodia through the freshly consolidating substance, and this, by 

 moulding itself upon the pseudopodia that issue from the orifices of the subjacent 

 surface, will itself be rendered tubular, and will continue to allow the passage of the 

 pseudopodia from the earliest chambers through the last formed layer of shell. And 

 I would suggest it as a subject for inquiry whether in the formation of dentine and 

 other calcified tubular tissues of higher animals, the tubular structure is not really the 

 result of the consolidation of an excretion-substance around filamentous prolongations 

 of the active protoplasmic substratum from which it is exuded. 



