4© Correlation of the Lower Silurian — Ulrich. 
Like beds XI, these also, mainly upon palasontological grounds, 
admit of division in two minor sections which I refer to as Xlla 
and XII(5. Of the 199 species mentioned in the list as occurring 
in beds XII, 113 are restricted to them, while only 5<S are com- 
mon to the upper and lower sections. Fifty-seven are restricted 
to Xlla-, and forty-six to Xllb. 
a. The thickness of this sub-division as seen in the hills sur- 
rounding the cities of Cincinnati, Covington and Newport is 
about 90 feet. In the lower 50 feet (exposed in the hills 270 
to 320 feet above low water mark in the Ohio river) there is 
often a considerable amount of sandy material, the limestone 
layers being nearly all impure; and some of the layers might 
even be described as fine grained sandstones. The intercalated 
shales, too, are less fine grained and of lighter color than usual. 
Fossils of many species, principally bryozoa, however, are mod- 
erately abundant and in a good state of preservation. The most 
distinctive forms are distributed about as follows: The five feet 
at the base (/. e. 270 to 275 feet above low water mark) hold 
great numbers of Callopora dalei, C. subplana^ and Peronopora 
vera, a species with larger cells than P. decipiens. The second 
of these species is restricted to this horizon, but the others 
range about fifty feet higher. 
Twenty feet above the C. stibplana bed many species make 
their appearance. Of these Heterotrypafrondosa, Dekayia as- 
pera and Hotnotrypa curvata may be mentioned as particularly 
characteristic of the horizon. In the succeeding twenty or 
twenty-five feet fossils are comparatively rare though several of 
the layers are sometimes charged with more or less worn ex- 
amples of Orthis tcsttidinaria. It is in this portion of the sec- 
tion that the sandy feature of the sub-division is the most 
conspicuous. Many of the layers show peculiar trails and 
fucoidal markings. 
The top of these layers is marked by a very persistent shell, 
the Streptorhynchus planoconvexus, which, though restricted 
to a vertical range of perhaps not over five feet, may be found 
wherever this horizon is exposed. 
The remaining portion of the sub-division, /. e. between 320 
and 360 feet above low water mark, furnishes the bulk of the 
rock used in constructing foundations at Cincinnati. Sub-crys- 
