MESOZOIC AND CENOZOIC LIFE 427 
most noticeable changes are seen (Figs. 247-249). The 
nautilus and ammonite groups of the Cephalopoda, 
with coiled shells, attamed great variety. Besides 
these shell-bearing forms, some have become divested 
of external shell, and like the squid and devil-fish of 
the present sea, have a partial internal skeleton in 
place of the shell covering (Fig. 247 (2)). 
Fic. 245. 
Ideal landscape of Jurassic period. (After Haushofer.) 
In some parts of the world, entire beds of rock are made of the 
remains of molluscan shells. There are, for instance, in central 
Texas, Cretaceous strata composed almost entirely of oyster shells 
(Fig. 250); in fact, the oyster group of bivalve shells is one of the 
most characteristic of the Mesozoic time. 
