The Early Stages of Bxtctrites. — Clarke. 37 
ty uniformly in going either to the north or to the south from 
the fulcrum point. 
The unique feature of this fault is the differential move- 
ment of the western limb with reference to the eastern one, 
and I therefore propose to call it a differential fold fault. 
Mild cases of this type of fault are probably by no means 
rare in mountain districts, but they would usually escape de- 
tection on account of the low angle of pitch. As more atten- 
tion is given to the pitch of folds in the study of mountain 
structure, it may be expected that pronounced instances of 
such faults will be occasionally discovered. 
THE EARLY STAGES OF BACTRITES. 
By John M. Clarke, Albany, N. Y. 
(Plate II.) 
In a recent number of this journal* the writer took occasion 
to describe the protoconch of a Devonian species of Orthoceras, 
remarkable for its fullness and rotundity, without evidence of 
distortion or shrinkage. In connection with this description 
reference was made to the similarity of form existing between 
this protoconch and that of the genus Bact rites, as repre- 
sented by specimens from the same geological horizon. 
The protoconch of Bactrites has been described by Brancof 
from specimens derived from the Wissenbach slates. This 
eminent eephalopodist mentions eight examples retaining this 
initial shell, in material which had been furnished to him by 
Privy-counselor E. Beyrich, and the figured examples show 
this protoconch to be an elongate, egg-shaped body, the shell- 
tube being attached at one of its extremities. The more com- 
plete of these figures, which is here reproduced in outline, 
shows this form as well as the narrowness and regular upward 
expansion of the shell-tube, and also the lateral position of 
the sipho, which has commonly been regarded as the principal 
determinative feature of the genus. 
The ovoid form of the protoconch is, as observed by 
Branco, unlike anything occurring in the Ammonitinee or the 
*American Geologist, vol mi. pp. 11'.' 115, Aug., 1893. 
[•Zeitschrift der deutsch. geol. Gesellsch. vol.37, pp. 1 '•». flgs. \.'l. 
1885. 
