626 SLUART, WHEELER: 
descriptions and illustrations of several species of fossils from the 
locality. 
The ‘ Back Bone”’ is a narrow, rocky ridge, rising about one 
hundred feet above river level and extending northward from 
the town of Grand Tower along the east bank of the Mississippi 
River for a distance of about one-half mile. It consists of strata 
of Devonian limestone, with beds which are probably of Lower 
Helderberg age below, dipping to the northeast at an angle of 
about.25°. @her< Bake Ovenkyus weally the northempendyor 
this ridge, but is isolated from the main ‘‘Back Bone” by an 
interval of several hundred feet. 
The field work upon which the present paper is based was 
done in August 1896. As the time which could be devoted to 
the work was limited, it was concentrated upon the north face of 
the “Bake Oven.” The section here studied is about 167 feet 
in thickness. An attempt was made to select the fossil-bearing 
zones in the section, and to make as complete collections from 
them as the time would allow. Twenty-six zones were recog- 
nized, eighteen of them being fossil-bearing to a greater or less 
extent. Additional time in the field would doubtless materially 
increase the number of species, and perhaps the number of zones, 
”) 
and a careful examination of the entire ‘‘ Back Bone’’ would be 
exceedingly valuable. 
DESCRIPTION OF THE SECTION, WITH LISTS OF FOSSILS. 
The field number given to the section was 5A. The beds, 
with their fossil contents, will be described, beginning with the 
lowest, 5A* and passing upward through the successive zones to 
5A”, which is the highest. The relative abundance of the 
species is designated as follows: (a) abundant, (c) common, 
(Ge), aves, 
5A* About fifteen feet of coarsely crystalline, gray limestone exposed, 
variable in texture, and often more or less arenaceous, especially near the 
top. Fossils numerous but not well preserved. 
1. Leptena rhomboidalis Wilck. (c). 
2. Chonetes laticosta Hall? (r). 
