97 
7.37 mm. [.29] ; height of Bast whorl 6.84 mm. [.23]; diameter 
of base 9.91 mm. [.39]. 
Orthoceras fustulosum. —Characterized by having the 
exterior covered with pustules of varying sizes and unequally 
distributed. 
Gomphoceras omicrok. —Rather large, compressed-ovate, 
abruptly constricted at aperture.; septa about 12—the last at 
largest diameter of shell—their distances increasing from 4.57 
mm. [-18] near apex to 8.13 mm. [.32] next to last chamber. 
Siphon large, sub-marginal. Length about 127 mm. [5.]; 
greater diameter about 76 mm. [3].; smaller 63 mm. [2.5]. 
Spirorbis ammox! —Aspect and size of & omphalodes 
Goldf., but more strictly planorbiform, has uniformly one more 
whorl, and is less rapidly enlarged. 
Spirorbis obesa.— Three or four times the diameter of S. 
omphalodes , consisting of two or three rapidly enlarging, 
nearly planorboid whorls. 
The foregoing is a summary of all that is at present known 
of the rocks of the Hamilton group on the western side of the 
lower peninsula of Michigan. 
A irar Arbor, 31 July, 1866. 
hLMrfwA 
