Prof. H. A. Nicholson—Organisms in Paleozoic Limestones. 23 
rounded or irregular nodules, which vary in diameter from less than 
a millimétre to more than a centimétre. The larger examples 
(Fig. 5, A) show a distinctly concentric structure, visible even to 
the naked eye, but the most powerful lens fails to show any obvious 
internal structure in fractured or weathered surfaces. Hxamined 
microscopically by means of thin sections, the nodules of Girvanella 
are seen to consist of exceedingly minute circular tubes, endlessly 
contorted and bent, and twisted together in loosely reticulate or 
vermiculate aggregations (Fig 5, B). The tubes vary in their size 
from 31; to =; of a millimétre in diameter. Most commonly they 
are about ;4; mill. in diameter. The walls of the tubes have a 
granular aspect, as if formed of exceedingly minute granules, but it 
is not possible to determine absolutely whether they are or are not 
truly calcareous in composition. No internal partitions are visible 
in the tubes, nor do they exhibit any perforations in their walls. 
A 
Fic. 5.—A, Fragment of limestone from the Ordovician rocks of Craighead, Girvan, 
of the natural size, showing numerous exceptionally large masses of Girvanella 
problematica, Nich. & Eth. jun. B, Section of a minute mass of Girvanella, 
enlarged about 60 times. 
In the original description of Girvanella problematica by Mr. R. 
Etheridge, Jun., and myself (loc. cit.), the genus was provisionally 
referred to the Rhizopoda, and was regarded as related to the 
arenaceous Foraminifera. This view of the affinities of Girvanella, 
from which I see no reason to depart, was the one taken by Mr. H. 
B. Brady, to whom we had submitted our specimens ; and this dis- 
tinguished authority compared Girvanella with the recent Hyperammina 
vagans, figures and descriptions of which have now been published 
(see Chall. Reports, vol. ix. p. 260, pl. xxiv. figs. 1-9). The same 
author’s great work on Foraminifera contains, however, another genus 
of arenaceous Foraminifera which admits, perhaps, of an even closer 
comparison with Girvanella than does the form above mentioned. I 
allude to the remarkable form described by Mr. Brady under the name 
of Syringammina fragilissima (Chall. Reports, vol. ix. p. 242, woodcut), 
in which the organism is free, and consists of a mass of minute 
arenaceous tubes disposed in concentric layers, and having a generally 
radiate arrangement. 
