178 
THE GEOLOGICAL MIDDLE AGE. 
the tendency to a coil is reduced to a single 
curve, so as to give the shell the outline of a 
horn, as in the Toxoceras (Figure 5) ; or the 
coil is entirely lost, and the shell reduced to 
its pi imitive straight form, as in the Baculites 
(Figure 6), which, 
except for their un- 
dulating partitions, 
might be mistaken 
for the Orthocera- 
tites of the Silurian and Devonian epochs. I have 
pi esented here but a few species of these extra- 
ordinary Cretaceous Ammonites, and, strange to 
say, with this breaking-up of the type into a num- 
bei of fantastic and often contorted shapes, it dis- 
appears. It is singular that forms so unusual 
and so contrary to the previous regularity of this 
group should accompany its last stage of exist- 
