450 Mr. R. Kiikpatrick on the Structure of 



One group of tlie Stromatoporoids, the Labechiidse, appear 

 to liave a somewhat different type of central chamber from 

 that of the rest, and the succeeding chambers are thinner- 

 walled and more vesicular. 



The sliarp longitudinal ridges of Beatricea have each a 

 little spiral system beneath the edge. 



To sum up : Stromatoporoids are adherent colony-forming 

 Perforate Foraminifera, each unit in one group of Stromato- 

 poroids consisting of a central and circumambient chamber 

 followed by spiral series of simple rather thick-walled 

 chambers, witli the walls perforated by pores and tubuli ; 

 here the growth is like that of the Imperforate genus Orhi- 

 toides (which has only vertical radial partitions, and not 

 horizontal ones in addition, as in OrhitoUtea) . 



In another group — the Labechiidaj — the central chamber 

 and immediately succeeding growth somewhat reminds one 

 of the Glohir/erina type. 



The Stromatoporoids are found in Ordovician, Silurian, and 

 Devonian strata. 



Eozoon. 



A vertical section of Eozoon shows an alternating series of 

 wavy bands varying in appearance according to the varying 

 mineral changes it has undergone. Commonly one finds 

 white zones of calcite and yellowish translucent zones of 

 olivine, which last may have undergone further changes into 

 gree)i serpentine. 



The white zones of dolomite or secondary calcite constitute 

 the "supplementary skeleton," and are seen to be abundantly 

 traversed by branching systems of canals. The yellowish or 

 greenish zones are occupied by horizontal rouleaux of minute 

 JMummulitid shells standing vertically, i. e. on their edges, 

 and closely pressed against one another. Sometimes the 

 peiipheral edges of neighbouring shells overlap, and there 

 may be here and there an appearance of a continuous spiral, 

 but in reality the arrangement is like that of a rouleau of 

 coins or flat disk-like beads strung on a string. Each shell 

 has a fuiniel-like umbilicus through which the main stolon 

 passes, and around which each coiled shell grows. In 

 addition, minute shells appear to be budded off promiscuously 

 from other stolon-like offshoots than the main central one. 

 Each rouleau forms a common supplemental chamber, which 

 communicates with its neighbours through circular orifices in 

 diaphragms. Each coiled shell (or coin of a rouleau) has 

 radiating septa and rows of pores in its thin primary wall. 



