Meteorology of the Ordovician. — Sardeso?i. 
389 
built by a colony of zooids or polypites. The zoarial stem and 
branches are nearly flat, becoming lens-shape in transverse 
section with age. They are made up of closely amalgamated 
tubes or cells which begin blindly at the medial part and open 
upon either surface of the zoarium, each being the work or 
skeleton of a zooid, the whole zoarium being the skeleton of a 
colony of zooids. The structure of the zoarium bears, in gen- 
eral, evidence of having been built by a closely incorporated 
colony, much like the gorgonids among corals. 
What is remarkable further is that the branches twist as 
they ascend and turn the faces of the zoarium thus from east to 
west. This could be explained as due to influence of the sun's 
rays upon the zooids. those that were in the most favorable 
position growing a little the faster and crowding their neigh- 
bors, and the crowding following the sun's course caused a 
twisting from east to west. A tendency to turn in that direc- 
tion would become finally inherent. This turning might be 
explained also as a device to prevent the repeatedly dividing 
branches from interfering,' but it appears to be not defined 
by that use since the twisting is least marked where the 
branching is most frequent as in the lower older part of the 
Figure 1. 
Explan. Fio. /. Rhinidictya mutabilis Ulrich, from the Ordovician 
Trenton strata at Saint Paul, Minnesota, a) Part of a zoarium show- 
ing branches twisting from left to right, b) Similar piece with verti- 
cal branch twisted and horizontal branch not twisted, c) A small spec- 
imen partly restored. 
