2/2 HISTORICAL PALAZSONTOLOGY. 
Upon the whole, the most characteristic of all the Creta- 
ceous Molluscs are the Cephalopods, represented by the remains 
of both Zetrabranchiate and Dibranchiate forms. Amongst the 
former, the long-lived genus JVautilus (fig. 201) again reap- 
Fig. 201.—Different views of Nazti/us Danicus. Faxée Limestone 
(Upper Cretaceous), Denmark. 
pears, with its involute shell, its capacious body-chamber, its 
simple septa between the air-chambers, and its nearly or quite 
central siphuncle. The majority of the chambered Cephalo- 
pods of the Cretaceous belong, however, to the complex and 
beautiful family of the Ammonztide, with their elaborately- 
folded and lobed septa and dorsally-placed siphuncle. ‘This 
family disappears wholly at the close of the Cretaceous period ; 
but its approaching extinction, so far from being signalised by 
any slow decrease and diminution in the number of specific 
or generic types, seems to have been attended by the develop- 
ment of whole series of new forms. The genus Ammonites 
itself, dating from the Carboniferous, has certainly passed its 
prime, but it is still represented by many species, and some of 
these attained enormous dimensions (two or three feet in 
diameter). The genus Azcyloceras (fig. 202), though likewise 
of more ancient origin (Jurassic), is nevertheless very charac- 
teristic of the Cretaceous. In this genus the first portion of 
the shell is in the form of a flat spiral, the coils of which are 
not in contact ; and its last portion is produced at a tangent, 
becoming ultimately bent back in the form of a crosier. Be- 
sides these pre- existent types, the Cretaceous rocks have 
yielded a great number of entirely new forms of the Ammmoni- 
tide, which are not known in any deposits of earlier or later 
date. Amongst the more important of these may be men- 
tioned Crioceras, Turrilites, Scaphites, Hamites, Ptychoceras, 
