276 FAMILY NUMMULINIDA. 



the closest approximation to Operculina ; and its relationship to that type is still more remark- 

 ably evidenced by the spreading-out of its last convolution. 



468. Geological Distribution. — There is no fact in Palaeontology more striking than the 

 sudden and enormous development of the Nummiilitic type in the early part of the 

 Tertiary period, and its almost equally sudden diminution, bordering on complete extinction. 

 The precise position of the immense beds of " Numraulitic limestone," the vast geographical 

 extent of which has been already sketched (1 451), has been a subject of much discussion ; 

 but the researches of M. D'Archiac, Sir R. Murchison, Sir Charles Lyell, and others, leave 

 no further doubt that these beds belong to the early part of the Tertiary period, and that 

 they correspond in position with the " Calcaire Grossier" of the Paris basin, and with the 

 " Bracklesham" and " Bagshot" beds of the London and Hampshire basins, in which 

 deposits alone are Nummulites found in the British islands. Although Nummulites have 

 been described as existing at periods anterior to this, it seems probable that such descrip- 

 tions have been founded on the occurrence of other helicoid Foraminifera bearing an incom- 

 plete resemblance to them. What could have been the conditions which so specially 

 favoured their production at the period in question, — which forced them (so to speak) to the 

 attainment of a size so uncommon among Foraminifera, — which occasioned the development of 

 such a multitude of varietal differences (some of these being limited to particular localities, 

 whilst others present themselves in nearly all the regions in which Nummulites abound), — and 

 which promoted and sustained the multiplication of individuals through a sufficiently long 

 succession of ages to cause a vast thickness of solid rock to be formed of little else than 

 their remains,* and what change in those conditions put a sudden and almost complete stop 

 to these operations, constitute most interesting subjects for physiological and geological 

 inquiry. Comparatively insignificant forms referable to N. jiilaimlata are the only examples 

 of this type which can be traced into the later Tertiary strata ; and these have continued 

 to maintain themselves to the present time through changes which have proved fatal to tlicir 

 gigantic congeners. 



Genus IV. — Polystomella (Plate XVI). 



469- Histon/. — Of the minute shells to which the generic name Puli/stomella is at present 

 assigned, one species, now known as P. crispa, secm.s to have early attracted the attention of 

 conchological observers and collectors, on account both of its beauty and of the frequency of 

 its occurrence ; having been described and figured more than a century ago by Plancus and 

 Gualtieri, and adopted by Linnreus under the designation Nautili's into his ' Systema Naturae.' 

 By this designation it continued to be generally known from the time of Linna;us to that of 

 Lamarck ; having been described and figured by Walker, Soldani, Fichtcl and Moll, Montague, 



* I have found, by the examination of numerous thin sections of Nummulitic limestones, that 

 the matrix wherein the entire Nummulites arc embedded is chiefly made up of more minute specimens 

 of the same types, and of comminuted fragments of the larger ones. 



