3o8 Ulrich on Correlation of the Lower Silurian. 
The best exposure of this subdivision known to mc is found 
in the Kentucky bank of the Ohio river, beginning about one 
mile west of the mouth of the Licking river. Here every layer 
of the upper 50 feet may be traced for nearly a mile, and, on 
account of the frequent washings to which the beds are sub- 
jected by the rising of the water in the river, many fine fossils. 
are annually collected there. 
At this locality the uppermost member of the series is formed 
by a crinoidal limestone layer, varying in thickness from i to 2 
feet and composed almost entirely of the comminuted remains 
of Hcterocrinus. The upper surface of this layer is rough and 
generally presents a mottled appearance due to small lenticular 
concretions, often charged with iron, which are contained in it. 
A large branching species of Ptilodlctya which I propose to 
name P. fonderosa., is apparently restricted to this layer. The 
branches of this bryozoan are often more than an inch in widths 
and nearly a quarter of an inch in thickness. 
Immediately beneath the heavy layers there is a little soft 
shale, and below this three or four feet of somewhat granular, 
unevenly bedded, limestones, which disintegrate rather rapidly 
and are disposed to break up into irregular layers on exposure to 
the weather. When freshly fractiu-ed, their crystalline interior 
is light blue in color, and contrasts strongly with their gray or 
brownish exterior. It is from them that the De7tdrocri7tus 
dyeri is derived. Specimens of this graceful crinoid are not 
rare, but, on account of the peculiar character of the rock, very 
few of them are well preserved. A singular feature relating to 
the occurrence of this crinoid is that the specimens are almost in- 
variably attached to the lower side of the layers. Other fossils 
are rare, and except a Ling7da (X. norivoodi jfames) always 
illy preserved. 
Below these layers there are about 12 feet of a soft clayey 
shale and more evenly bedded impure limestones, when a layer 
holding numerous hard cla}'^ nodules is met with. Some of the 
limestones are highly fossiliferous, while the shale in places 
contains numerous fragments of MoJiotry fella brlareus. Other 
fossils that are more or less abundant at this horizon ^lXq. Modiol- 
opsis cincitinatiensis, M'? canccllafa^ Lyrodesma plam/m^ Pleii- 
rotomaria ohioensis^ Bellerofhon bilobatus., Lockeia seliquariay 
