CHAPTER X. 



FAMILY NUMMULINIDA. 



413. The Nummulites and their alhes, as they are the most gigantic, are also the most 

 highl}^ developed representatives of the Foraminiferous group : their structure presenting 

 in every particular an advance upon that of the types already described. It is in this 

 family that the shell attains its greatest density, that of some genera being of an almost 

 ivory-like toughness ; and this character seems in some way related to the exceeding 

 fineness of its tubularity, being most pronounced in those cases in which the tubuli 

 are most minutely and most regularly disposed. It is here, too, that we find it to be a rule 

 without exception, that the septal plane is completely differentiated from the rest of the shell, 

 the ordinary tubular structure being deficient, and no other passages than the principal 

 aperture existing in the septum, except a few large " orbuline" pores. Again, it is here a rule 

 almost without exception that the wall of each chamber is complete in itself, instead of being a 

 mere " tent ;" hence each septum consists of two laminae, one belonging to each of the cham- 

 bers which it divides ; and between these laminae is interposed a set of radiating canals, which 

 communicate with a system that passes along the successive turns of the spiral or the annuli 

 of the cyclical disk — according to the plan of growth. The development of this canal-system 

 is related to that of the " intermediate" or "supplemental skeleton," which is here in almost 

 every instance superadded to the proper walls of the chambers, consolidating into one compact 

 fabric what would otherwise be a series of slightly connected segments. 



414. The typical plan of growth in this family is a symmetrical spiral, with progressive 

 increase in the breadth of its convolutions, giving to the shell more or less of the character 

 of that of a Nautilus or an Ammonite. And, as a general rule, each convolution invests the whole 

 of the preceding on both sides ; the spiral lamina of shell being continued to the umbilicus, even 

 where it so closely adheres to that which it overlies as to leave no space whatever for the 

 extension of the soft body between the two. In the typical Niimmidin(P, however, each 

 segment of the body gives off a pair of alar lobes (^ 71), which prolong themselves to the 

 umbilicus in alar extensions of the chamber which it occupies. These alar lobes, moreover, 

 in certain Xtimmulinida, are detached by intervening septa from the segments to which they 

 seem properly to belong, and may even be broken up into small sub-segments, so as to form 

 on either side of the spire an overgrowth of secondary chamberlcts which bears a strong 

 analogy to that of Tinoporus. Even in those genera, however, in which the complete invest- 



