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 (^Campanulariajohnstonia 1 ), there do not appear to beany 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 tilled by quartz crystals, grown 



Fig. 3.— Cannorhiza connexa Haeekel. Adradial section, tig, gelatinous umbrella; gc, central Btomach : gg, bottom 

 of the central stomach (gastrogenital membrane, with the genitalia, s); ir, subgenital porticus; ah, brachiferoue 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. 



' A Monograph of the Gymnoblastic or Tubnlarian Hydroids, by George James Allman (a pub- 

 lication of the Ray Society), Part I, 1871. p. 23, fig. 2. 



