﻿NUMMULINA. 



149 



Nummulina pristina, Brady. Plate XI, figs. 8 — 11. 



Nummulina pristina, Brady, 187-1. Ann. and Mag. Nat. Hist., ser. iv, vol. xiii, 



p. 225, pi. xii. 



Characters. — Shell lenticular, obscurely radiate, with angular or more commonly 

 blunt and rounded margin. Composed of three to four convolutions gradually increasing 

 in width, the third convolution having about fifteen chambers. Primordial chamber of 

 medium size. Diameter inch (0 - 85 mm.) ; thickness inch (0-36 mm.). 



The brief zoological description of this little Nummulite above given represents the 

 average characters of the comparatively few specimens that have hitherto been met with. 

 In some particulars further information may be gained by the detailed examination of 

 individual specimens. 



The dimensions above given are those of one of the larger examples. Most of the 

 specimens are bilaterally symmetrical or nearly so, white and smooth as to surface, the 

 uniformity being broken only by radial lines more transparent in texture than the rest of 

 the shell. A section on the median plane reveals a spiral of three or four convolutions, 

 the whorls of nearly equal width or only increasing slightly towards the periphery, a 

 primordial chamber relatively rather large, the ordinary chambers few in number for a 

 Nummulite, and bounded by curved septa. 



The relation between the diameter and thickness is apparently tolerably constant, 

 that is, about as 2 \ to 1 ; larger examples, however, exhibit some tendency to spread out 

 and grow thinner at the periphery. When the surface of the test is not worn, the 

 radiation is either very indistinct or appears in the form of uneven, slightly curved lines 

 of somewhat darker colour, but without sensible limbation ; but in weathered specimens 

 not only are the lines more or less elevated, but the centre from which they proceed is 

 thickened and the test becomes to some extent umbonate also. 



An accidentally split specimen (PL XI, fig. 9) will serve the purpose of a horizontal 

 section. It consists of three convolutions, the outermost having sixteen chambers, and 

 the second twelve or thirteen. Another somewhat larger individual has precisely 

 similar septation, so that, without assigning any great importance to it, the drawing may 

 be assumed to represent a specimen with about the normal number of chambers for the 

 adult condition. 



The primordial chamber has been measured in three examples, and the diameter 

 found to be -004, -003, and -0027 of an inch (0'1, 0-08, and 0'07 mm.), being 

 respectively from \ to yq of the entire diameter of the test. 



The minute tubulation of the shell is perfectly preserved, and may be easily seen in 

 the transverse section under a magnifying power of 100 diameters, as in PI. XI, fig. 10. 



