FORAMLNIFERA AND RADIOLARIA. 



91 



FIG. 330. 



interior, instead of being minutely divided into cnamberlets, is found to 

 consist of a regular series of simple chambers; while from each of these 

 proceeds a pair of elongated extensions, which correspond to the 'alar 

 prolongations' of other spirally growing Foraminifera ( 486), but which, 

 instead of wrapping round the preceding whorls, are prolonged in the 

 direction of the axis of the spire, 

 those of each whorl projecting be- 

 yond those of the preceding, so that 

 the shell is elongated with every in- 

 crease in its diameter. Thus it ap- 

 pears that in its general plan of 

 growth, Fusulina bears much the 

 same relation to a symmetrical Eo- 

 taline or Nummuline shell, that 

 AlveoUna bears to Orbiculina ; and 

 this view of its affinities is fully con- 

 firmed by the Author's microscopic 

 examination of the structure of its 

 shell. For although the Fusulina- 

 Limestone of Russia has undergone 

 a degree of metamorphism, which 

 so far obscures the tubularity of its 

 component shells, as to prevent him 

 from confidently affirming it, yet 

 the appearances he could distin- 

 guish were decidedly in its favor. 

 And having since received speci- 

 mens from the Upper Coal Mea- 

 sures of Iowa, U. S., which are in a much more perfect state of pre- 

 servation, he is able to state with certainty, not only that Fusulina is 

 tubular, but that its tubulation is of the large coarse nature that marks 

 its affinity rather to the Rotaline than to the Nummuline series. This 

 type is of peculiar interest, as having long been regarded as the oldest 

 form of Foraminifera which was known to have occurred in sufficient 

 abundance to form Eocks by the aggregation of its individuals. It will 

 be presently shown, however, that in point both of antiquity and of 

 importance, it is far surpassed by another ( 493). 



486. Nummulinida. All the most elaborately constructed, and the 

 greater part of the largest, of the ' vitreous ' Foraminifera belong to the 

 group of which the well-known Nummulite may be taken as the repre- 

 sentative. Various plans of growth prevail in the family; but its distin- 

 guishing characters consist in the completeness of the wall that surrounds 

 each segment of the body (the septa being double instead of single as 

 elsewhere), the density and fine porosity of the shell-substance, and the 

 presence of an ' intermediate skeleton,' with a 'canal-system' for its 

 nutrition. It is true that these characters are also exhibited in the high- 

 est of the Eotaline series ( 484), whilst they are deficient in the genus 

 Amphistegina, which connects the Nummuline series with the Eotaline; 

 but the occurrence of such modifications in their border-forms is common 

 to other truly Natural groups. With the exception of Amphistegiria, all 

 the genera* of this family are symmetrical in form; the spire being nauti- 

 loid in such as follow that plan of growth, whilst in those which follow 

 the cyclical plan there is a constant equality on the two sides of the 

 median plane: but in Amphistegina there is a reversion to the rotalian 



Section of Rotalia Schroetteriana near its 

 base and parallel to it; showing <*, a, the radiat- 

 ing interseptal canals; b, their Internal bifurca- 

 tions; c, a transverse branch; d, tubular wall of 

 the chambers. 



