284 MANUAL OF ZOOLOGY. 



lopoda, the Tetmbranchiata is the oldest, attaining its maximum 

 in the Palaeozoic period, decreasing in the Mesozoic and 

 Kainozoic epochs, and being represented at the present day by 

 the single form Nautilus pompilius. Of the sections of this 

 order, the Nautilidce proper and the Orthoceratidce are pre- 

 eminently Palaeozoic, and the Ammonitidce is not only pre- 

 eminently but is almost exclusively Secondary. Of the abun- 

 dance of the two former families in the Silurian seas some 

 idea may be obtained, when it is mentioned that about a 

 thousand species have been described by 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 Orthoceratidce died out much sooner, being ex- 

 clusively Palaeozoic, with the exception of the genera Ortho- 

 ceras itself and Cyrtoceras, which survived into the commence- 

 ment of the Secondary period, finally dying out in the Trias. 



The second family of the Tetrabranchiata, viz. the Ammoni- 

 tidce, is almost exclusively Secondary, being very largely re- 

 presented by numerous species of the genera Ammonites, Cera- 

 tites, Baculites, Turrilites, &c. The only Palaeozoic genera are 

 Goniatites and Bactrites, of which the former is found from 

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

 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 Ptyclioceras 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 Belemnitidce. Some 

 of the Teuthidce and Sepiadce 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. 



