76 THE OPEN BOOK OF NATURE 
roads were made in the neighbourhood of Peter- 
borough, where I then lived. People used to tell 
me that they were “ thunder-bolts,” a statement 
which aroused my curiosity greatly, but about which 
I was sceptical. In time I learned that these curious 
fossils are the remains of extinct cuttle-fish, being 
their “‘ pens,” or internal shells. I used to find am- 
monites as well, and was told that they were petrified 
snakes. ‘This, however, is not the case, for in truth 
ammonites are fossil Cephalopods closely related to 
the modern pearly nautilus, which is found in the 
Pacific and Indian Oceans. The shell of the nautilus 
has many chambers, but the 
animal lives-in the last and lar- 
gest ; as one might say, it views 
life through the front door. The 
remaining chambers are filled with 
gas, which reduces the animal’s 
weight in water, and enables it 
to crawl or swim the more easily. 
The few species of nautilus which 
now exist are the only surviving 
relatives of the numerous ancient 
ammonites and nautili, Ammo- 
nites flourished exceedingly in the 
Mesozoic Era, and their remains 
are characteristic of the Mesozoic 
rocks in which many species are 
found, some of them of great 
size, being about five feet in dia- 
meter. But the majority are comparatively small. 
Plenty of ammonites can be got in the English chalk. 
The genus Nautilus had ancestors as far back as Silu- 
Fie. 25. — Orrno- 
CERAS ANNULA- 
TUM (SILURIAN). | 
