EE. Wethered—Structure of Jurassic Pisolite. hog 
INFERENCE. 
I now come to the inference which is to be drawn from the 
observations I have made. ‘The first conclusion, which cannot be 
disputed, is, that we can no longer regard the pisolites as a form of 
concretionary oolitic structure. The so-called pisolite granules are 
really formed by the growth of an organism around a nucleus. As to 
the determination of that organism the similarity to the genus Gir- 
vanella is at once suggested. This organism was first described by 
Professor A. Nicholson and Mr. R. Etheridge, jun.,' and has since been 
more fully referred to by Professor Nicholson.? He remarks, “This 
curious fossil occurs in great numbers in the lime-formation of the 
rock,” the Ordovician limestone, “and presents itself in the form of 
small rounded or irregular nodules which vary in diameter from less 
than a millimetre to more than a centimetre. The larger examples 
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. Examined microscopically the 
nodules of Girvanella are seen to consist of exceedingly minute 
circular tubes, endlessly contorted and bent, and twisted together in 
loosely reticulated or vermiculate aggregations.” This description 
of the microscopic structure of Girvanella problematica, the one 
species of the genus mentioned by Professor Nicholson and Mr. 
Etheridge, jun., corresponds very closely with the structure of the 
Coralline pisolites, and with the minute tubuli in some of the Pea- 
grit spherules at the base of the Inferior Oolite. A comparison of 
actual specimens confirms the similarity, and I have no hesitation 
in referring the Coralline pisolites to the genus Girvanella. Nor do 
I find sufficient evidence to warrant me in making a new species. 
It is true that the majority of the tubes are smaller than those in 
G. problematica, and that Professor Nicholson makes no reference to 
a nucleus around which they congregate. Now, the first portion of 
a Jurassic Girvanella spherule to undergo change is the nucleus. 
In some specimens we find simply crystalline calcite into which the 
original nucleus has been converted. The Silurian rocks, in which 
Girvanella was first discovered, are so much older than the Jurassic 
Oolite that it is not surprising to find the original nucleus changed 
into crystalline calcite. In the specimens which I have examined 
of G. problematica there seems to me to be undoubted signs of a 
nucleus, and I therefore do not see my way to make the absence of 
a clearly defined nucleus in the Silurian forms the reason for proposing 
another species of those in the Coralline Oolite, but it is possible 
that at some future time evidence may be obtained which would 
warrant that course being taken for reasons apart from the nucleus. 
With some of the spherules of the Pea-grit the matter is different. 
The larger well-defined tubuli represented in Figs. 10 and 11 are 
wanting in the Coralline forms, and I therefore propose to give 
them the name of Girvanella pisolitica. It is a question whether 
1 Mon. Sil. Foss. Girvan, pp. 22-3. pl. ix. 
2 Geou. Mae. N.S. Dec. ILI. Vol. V. p. 22, 1888. 
