THE GEOLOGICAL MIDDLE AGE. 
177 
traced the Chambered Shells from the straight, 
simple ones of the earliest epochs up to the in- 
tricate and closely coiled forms of the Jurassic 
epoch. In the so-called Portland stone, belonging 
to the upper set of Jurassic beds, there is only 
one type of Ammoniie ; but in the Cretaceous 
beds, immediately above it, there set in a number 
of different genera and distinct species, including 
the most fantastic and seemingly abnormal forms. 
It is as if the close coil by which these shells had 
been characterized during the 
Middle Age had been sudden- 
ly broken up and decomposed 
into an endless variety of out- 
lines. Some of these new types 
still retain the coil, but the 
whorls are much less compact 
than before, as in the Crioceras 
( Figure 1) ; in others, the di- 
rection of the coil is so changed as 
to make a spiral, as in the Tur- 
rilites (Figure 2) ; or the shell 
starts with a coil, then proceeds in 
a straight line, and changes to a 
curve again at the other extremity, 
as in the Ancyloceras (Figure 3), 
or in the Scaphites (Figure 4), in 
which the first coil is somewhat 
closer than in the Ancyloceras ; or 
