2/2 HISTORICAL PALEONTOLOGY. 



Upon the whole, the most characteristic of all the Creta- 

 ceous Molluscs are the Cephalopods, represented by the remains 

 of both Tetrabranchiate and Dibratichiate forms. Amongst the 

 former, the long-lived genus Nautilus (fig. 201) again reap- 



Fig. 201. Different views of Nautilus Danicus. Faxoe 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 Ammonitidce, 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, biit it is still represented by many species, and some of 

 these attained enormous dimensions (two or three feet in 

 diameter). The genus Ancyloceras (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 Ammoni- 

 tidce, 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, 



