EE ee Royal Microscopical Society. 183 
No. 1. Pattern regular, quincuncial, beads appearing about 
1—4” diameter, interspaces double that of the beads’ diameters. 
No. 2. Pattern regular, quincuncial, beads about half the size, 
and interspaces about half those of No. 1. 
No. 3. Beads appearing about 1—7" in diameter and touching. 
This pattern readily gives the hexagonal aspect obtainable with 
many diatoms. 
No. 4. Regular rectangular pattern, beads appearing about 
1—20" in diameter. 
No. 5. Similar beads to 4, grouped very irregularly with wide 
irregular interspaces, in which no beads could be discerned. 
No, 6. Similar beads in rows wider apart in one direction than 
in another, the interspaces being unequal in rectangular directions 
and many times the diameter of the beads. 
No. 7. Rather larger beads irregularly arranged, some lines 
straight, and others curved. 
No. 8. Large beads appearing from 1—8” to 1—7” in diameter 
surrounded by beads a quarter their size. 
No. 9. Large composite bead, as if composed of three adhering 
and compressed beads, surrounded with smaller beads irregularly 
arranged, and the interspaces filled with innumerable minute beads. 
No. 10. Large bead, surrounded by concentric circles of smaller 
beads. 
No. 11. Small beads in concentric rows, like a fragment of a 
delicate circular diatom. 
No. 12. Beading so fine and close as not to admit of distinct 
separation with mag. 1000. 
No. 13. Beading only indicated, and distinguished from clear- 
looking spaces somewhat as a surface of dead-gold is distinguished 
from a burnished one. 
No. 14. A form approximating to some of the polycistina, but 
with large beads as well as holes. 
The above by no means represent all the varieties noticeable 
in a single slide. In one case large beads appeared built up of 
smaller ones, but the optical appearances were too puzzling to say 
exactly what the true form was. It seems probable that the clear 
spaces only differ from the beaded ones in the fact that the silex, 
though precipitated in spherules, has formed a pattern too minute 
to be discerned. 
An examination of the artificial diatoms shows that purely 
chemical and physical considerations will account for the varieties 
of pattern we notice in natural diatoms, and their living structure 
appears only to provide the conditions under which the silicious 
precipitation takes place, according to the ordinary laws of chemical 
action and molecular coalescence. 
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