of the Lower Malm of Aargau. 53 
The rounded impressions on the outer surface, which are 
only present in the almost homogeneous glass-like specimens, 
are not visible in transmitted light when the object is mounted 
in balsam. On the contrary, the sandy grains (which when 
the shells are viewed as opaque seem to be absent) become 
thus plainly visible with the aid of a high power and the pola- 
riscope. . 
In none of the sections from the Upper Argovian limestones 
of the Geissberg, where the cement is calcareous, are any 
traces of this peculiar condition of the surface visible. 
When found in material much changed through the action 
of atmospheric agencies, it becomes difficult to distinguish the 
shells from Sperdllina without the use of a high power. 
The condition of the siliceous tests differs widely from those 
changed by pseudomorphoses ; so that it is evident that they 
represent their natural state. It can easily be accounted for 
by the great amount of silica, as may be judged from the 
immense accumulations of siliceous sponges in the same beds, 
whilst these are absent in the beds with calcareous Trocham- 
mine, although siliceous tubes are also met with in strata 
with hardly any siliceous substances, except grains of quartz- 
sand and argillaceous compounds, as some Liassic Involutine 
and the Zrochammine of the Bathonian limestones of the 
Frickthal. 
As typical specimens of this variety may be considered 
those resembling in the mode of growth the regularly con- 
voluted recent variety, described as Spirdllina arenacea, Will., 
but with a test resembling somewhat in its microscopical 
structure Involutina silicea, Terg.* A similar form was 
described by Giimbelt from the same zone at Streitberg as 
Spirillina tenuissima, and from the Hickberg by Kiibler and 
Zwingh{t (also from the Bathonian and Argovian II.). 
Similar regularly convoluted discoidal forms are known from 
many other formations ; but the composition of the test is 
generally somewhat different, as in Cornuspira Hernest, 
Karrer$§, from the Vienna basin, in Serpula Ressleri 
Schmid ||, from the German Zechstein, and in Z. incerta 
from the English Permian rocks {. 
* Terquem, Second Mém. For. Lias, p. 450, pl. vi. fig. 11. 
t+ Gumbel, Die Streitherger Schwammlager &c. p. 214, pl. iv. fig. 12, 
} Kubler & Zwingl, Die Foraminiferen des Schweizer Jura, p. 19, 
Ie tige 2. 
§ Water, “Ueber das Auftreten &c.” (Sitzungsh. k, Akad. Wiss. 
Wien, vol. lii. p. 495, pl. i. fig. 10). 
|| Schmid, Neues Jahrb. Min. 1867, p. 582, pl. vi. figs. 46, 47, 
q Jones, Parker, & Kirkby, Ann. & Mag. Nat. Hist. ser. 4, vol. iv. 
p. 388, pl. xii. fig. 1. 
