MIDDLE CAMBRIAN. 11 
Beyond the appearance of the surface of many of the complex, irregular 
forms of Laotira cambria, which suggests the anastomosing base of some of 
the hydroids (Campanularia johnstonia'), there do not appear to be any points 
of comparison between the Middle Cambrian forms and the hydroids. 
LITHOLOGIC CHARACTERS OF THE SILICEOUS NODULES. 
A thin section from a nodule containing fragments of the trilobite 
Olenoides curticei, when examined under the microscope, shows that the 
space once occupied by the test is now filled by quartz crystals, grown 
Fic. 3.—Cannorhiza connexa Haeckel. Adradial section. ug, gelatinous umbrella; gc, central stomach; gg, bottom 
of the central stomach (gastrogenital membrane, with the genitalia, s); ir, subgenital porticus; ah, brachiferous plate; 
ap, arm pillars; cd, pillar canals; ga, buccal stomach; ab, oral arms (adradial); cb, brachial canals; an, funnel frills (suck- 
ing mouths). 
from the matrix toward the center of the spaces. Lines of opaque par- 
ticles indicate the former presence of the test. ‘The sections of the trilobite 
tests are well defined, and the quartz crystals are much larger than those 
in the body of the nodule. 
At my request, Prof. Joseph P. Iddings and Dr. C. Willard Hayes both 
studied the microscopic characters of the nodules, and they arrived at 
essentially the same conclusion in regard to them. 
1 A Monograph of the Gymnoblastic or Tubularian Hydroids, by George James Allman (a pub- 
lication of the Ray Society), Part I, 1871, p. 23, fig. 2. 
