230 



HYDR0Z0A. 



spicuous concentric laminae but in a series of vertical calcareous 

 rods ("radial pillars"), which are directed at right angles to the 

 laminae. The "laminae," in fact, are not continuous calcareous 

 layers, but are in most cases really formed by numerous irregular 

 calcareous rods or processes which are given out by the " pillars " at 

 definite levels, and which join with one another and with the pillars 

 to form a reticulated or porous membrane. Hence, in a typical 

 Stromatoporoid, the last-formed concentric lamina, which consti- 

 tutes the outer surface of the organism, exhibits numerous minute 

 pores, which open internally into the uppermost " interlaminar 



Fig. 114. — A specimen of a Stromatoporoid, of the natural size, showing the concentrically 

 laminated skeleton. The under surface is concave and was probably attached to a foreign body. 

 From the Trenton Limestone of Canada. (After Billings.) 



space," and which probably served to give exit to the zooids of the 

 colony. The outer layer also commonly shows numerous small 

 granules or tubercles, which are really the free upper ends of the 

 vertical or radial pillars. These last-named structures are small cal- 

 careous columns, which have a general radial direction, and which 

 cross the interlaminar spaces vertically and thus connect successive 

 concentric laminae with one another. In some cases (as in the 

 genus Clathrodictyon) the pillars are merely short perpendicular 

 processes which run from the upper surface of one lamina to the 

 under surface of the next. In other cases (as in the genus Acti?i- 

 ostroma, fig. 1 15, b and c) the pillars run continuously through a con- 

 siderable number of laminae. In many cases, it can also be shown 



