TETRABRANCHIATE CEPHALOPODS. 59 



M. Barrande from the Silurian basin of Bohemia alone. The 

 Nautilidce proper have gradually decreased in numbers from 

 the Palaeozoic, through the Secondary and Tertiary periods, 

 to the present day. The Ortlioceratidce died out much sooner, 

 being exclusively Palaeozoic, with the exception of the genera 

 Ortlioceras itself and Cyrtoceras, which survived into the com- 

 mencement of the Secondary period, finally dying out in 

 the Trias. 



The second family of the Tdrdbranchiata — viz., the Ammio- 

 nitidce — is almost exclusively Secondary, being very largely 

 represented by numerous species of the genera Ammonites, 

 Ceratites, Bamdites, Tiirrilites, &c. The chief Palaeozoic genera 

 are Goniatites and Bactrites, of which the former is found 

 from the Upper Silurian to the Trias, whilst the latter is a 

 Silurian and Devonian form. The genus Ammonites, how- 

 ever, occurs in the Carboniferous, and has been also found in 

 strata of Tertiary age. The genus Ceratites is characteristi- 

 cally Triassic, but it is said to occur in the Devonian rocks, 

 and some species are Cretaceous. All the remaining genera 

 are exclusively Secondary, the genera Baculites, Turrilitcs, 

 Hamites, and Ptychoccras being confined to the Cretaceous 

 period. 



Of the Dibranchiate Cephalopods the record is less perfect, 

 as they have few structures which are capable of preserva- 

 tion. 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 

 Belemnitidce. Some of the Teuthidce and ScpiadcG are found 

 both in the Secondary and in the Tertiary rocks, and two 

 species of Argonaut have been discovered in the later Ter- 

 tiaries. No example of a Dibranchiate Cephalopod is known 

 from the Palieozoic deposits, and the order attains its maxi- 

 mum at the present day. 



Tetrabeanchiate Cephalopods. 



The Tetrabranchiate Cephalopods are characterised by being 

 creeping animals, protected hy an external, many-cliam'bcred shell, 

 the septa hetwccn the chawMrs of v)hich are perforcdcd hy a mem- 



