TWO GIGANTIC TYPES OF AEENACEOUS FOEAMINIFEEA. 
749 
much greater effect in obliterating its structural characters, and even in obscuring their 
outlines, than the same process when a different material is concerned. Thus the 
Calcareous test of ParJceria becomes almost devoid of character when its cavities are 
filled with a subcrystalline calcareous mineral ; whilst a specimen having its chambers 
occupied by silex has lost none of its distinctive characters. Chemical analysis of the 
infiltrated fossil ( Loftusia ) shows that at least 99 per cent, consists of Carbonate of 
Calcium, the remaining 1 per cent, being chiefly siliceous matter, a composition repre- 
senting equally well an average sample of the limestone matrix. The test, therefore, is 
built up of Calcareous sand-grains, incorporated by a cement of carbonate of calcium. 
But although the selective power which seems traceable in some Arenaceous Forami- 
nifera, enabling them to choose certain sand-grains in preference to others*, has no 
exercise in respect to the chemical nature of the constituents of the test, there is still 
something of the same sort observable in relation to the size and distribution of the par- 
ticles which go to its formation. Thus whilst the septa and the looser portions of the 
labyrinthic structures are coarsely arenaceous, the spiral lamina is composed of exceed- 
ingly minute particles. The presence of a number of specimens of the smaller species 
of Foraminifera imbedded by accident with the sand in which they were living, and 
now forming a portion of the fabric (Plate LXXX. fig. 3 renders it comparatively 
easy to estimate the size of the sand-grains. The largest that could be satisfactorily 
measured was about ythj of an inch in diameter ; but they seldom attain more than one 
half this size. Specimens of the same species abounding in the Limestone matrix run 
to much larger dimensions. One of the almond-shaped specimens of Loftusia which 
appears to have had its walls somewhat disintegrated by pressure or otherwise, presents 
its constituent granules in a very uniform condition, both as to their general appearance 
and their dimensions, and in this instance the average diameter is about -j^<y of an inch. 
In the more compact portions of the labyrinthic system, the granules are smaller; and 
as they approach the spiral lamina they become still more minute. In the thin layer 
which constitutes the lamina itself, a magnifying-power of 500 diameters (the highest I 
have been able to use with advantage on any section yet prepared) shows the ultimate 
structure only as a uniform, densely packed mass of particles, individually too small for 
even approximate measurement. The transparent sections (Plate LXXIX. figs. 4 & 5) 
cross regions in which the constituent granules are exceptionally small and uniform, 
and show well their close setting in the compact portions ( sg , sg). 
45. The variable external appearance of such of the fossils as have been exposed — 
through the unequal action of the weather, especially on the fractured surfaces — is pro- 
* The question of selection of materials amongst Foraminifera with composite tests is one to which my 
attention was drawn in a recent chemical examination of subarenaceous Milioloe ( Quinqueloculina agglutinans) 
obtained by Mr. Jeffreys from deep water in the Hebrides. Their tests were clearly formed of sand-grains 
and cement. They occurred in siliceous sand, having but a very small percentage of calcareous matter derived 
from the debris of Molluscan and other shells ; yet they were entirely soluble in very weak acid, leaving scarcely 
a microscopic trace of silica. 
