246 The American Geologist. October, 1905 
Dinorthis retrorsa and Leptaena rhomboidalis horizon is 
Rhynchotrema dentata, or, at any rate, a scarcely distin- 
guishable variety of this species. It occurs at this horizon 
at Arnheim, Ohio, and near Maysville, Kentucky. Farther 
southward, in the absence of Dinorthis retrorsa, it may be 
traced along the same horizon, in association with Leptaena 
rhomboidalis . 
Rynchotrema dentata occurs associated with Lep- 
taena rhomboidalis at the Brown's run school house, about 
8 miles southeast of Maysville, two miles northeast of Rec- 
torville ; also 30 miles south of Maysville, at the foot of the 
hill east of Wyoming; 14 miles southwest of Wyoming, 
about half a mile southwest of Howard mill, on the road 
over the hill to Spencer; 13 miles southwest of Howard 
mill, at the Curry bridge across Howard creek, reached by 
going a mile south, and then two miles west from Indian 
Fields. A mile north of the crossing of the Stanford-Row- 
land pike over Logan creek, at a large exposure on the west- 
ern side of the creek, Leptaena rhomboidalis was found in 
the upper part of the exposure. Rhynchotrema dentata is 
found associated with Leptaena rhomboidalis also a mile 
and three-quarters west of the court house at Lebanon, Ken- 
tucky, near the lower part of a gully located 20 degrees west 
of north from the home of Col. J. B. Wathen ; also 30 miles 
northwest of Lebanon, half a mile north of High Grove, and 
about six miles south of Mount Washington. 
Leptaena rhomboidalis may be found at many localities 
at which Dinorthis retrorsa and Rhynchotrema dentata are 
absent. This horizon in the Arnheim bed is fairly fossili- 
ferous at all localities in Ohio, Indiana, and Kentucky. On 
this account it may be traced with confidence entirely 
around the Ordovician area of Ohio, Indiana, and Kentucky. 
It promises to be a very valuable horizon for purposes of 
mapping, especially for those who are not familiar with the 
bryozoans of this area, the bryozoans, after all, constituting 
the final authority in the discrimination of Ordovician strata. 
Rhynchotrema dentata is found in association with Lep- 
taena rhomboidalis and Dinorthis retrorsa also at Clifton,. 
Tennessee. 
On the eastern side of the Cincinnati geanticline. 
