338 MANUAL OF ZOOLOGY. 



Ceratites, Baculites, Turrilites, &c. .The only Palaeozoic genera 

 are Gomatites and Bactrites, of which the former is found from 

 the Upper Silurian to the Trias, whilst the latter is a Devo- 



Fig. 122. — shells of Secondary Cephalopods. t Ancyloceras Mnther07tianusi 2 Sca- 

 phites agualis ; 3 Crioceras Duvalii ; \ Hamites attenuatusi s Turrilites cate- 

 ttatus, 



nian form. The genus Ceratites is characteristically Triassic, 

 but it is said to occur in the Devonian rocks. All the remain- 

 ing genera are exclusively Secondary, the genera Baculites, 

 Turrilites, Hamites, and Ptychoceras being confined to the Cre- 

 taceous period. 



Of the Dibranchiate Cephalopoda the record is less perfect, 

 as they have few structures which are capable of preservation. 

 They attain their maximum, as fossils, shortly after their first 

 appearance in the Secondary rocks, where they are represented 

 by the large and important family of the Bdemnitidce. Some 

 of the TeuthidcB and Sepiadm are found both in the Secondary 

 and in the Tertiary rocks, and two species of Argonaut have 

 been discovered in the later Tertiaries. No example of a 

 Dibranchiate Cephalopod is known from the Palaeozoic de- 

 posits, and the order attains its maximum at the present day. 



