236 
The American Geologist. 
October, 1893 
taking place. On the other hand, the change in the character 
of the siphuncle here described might be claimed as simply a 
return, later in life, to ancestral characters. To determine 
this question it will be necessary to make a much more gen- 
eral study of the charactersof the siphuncle in earlier (Middle 
Silurian) species than has yet been done. 
In a distinct species (fig. 4) with a smooth shell, from the 
Clinton, at Todd's Fork, north of Wilmington, Ohio, the sub- 
central portion is more constant, and the width of the 
siphuncle increases with that of the shell, as it normally 
should do. 
In a third species, Orthoceras ignotum Foerste, 6, the char- 
acters of the siphuncle are quite constant. It is cylindrical, 
5, 7, and always moderately excentric, as in 7 ; figure 5 is cut 
at right angles to the plane passing through the siphuncle and 
the center of the shell, and so does not show this excentricitj 7 . 
The chamber cavities are always higher, and the apical angle 
lower than in O. erraticum so that the shell can be distinguished 
easily, even b} r fragments from the upper end. It forms a 
good instance of a species with the character of the siphuncle 
constant throughout. It is found at Hanover, Indiana, and at 
many localities of the Clinton of Ohio, especially at Huffman's 
quarry, near Dayton, Ohio. 
