314 



MANUAL OF ZOOLOGY. 



Cephalopoda. The Cephalopods are largely represented in 

 all the primary groups of stratified rocks from the Lower 

 Silurian up to the present day. Of the two orders of Cepha- 

 lopoda, the Tetrabranchiata 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 Nautilidcc proper and the Orthoceratidcz are pre- 

 eminently Palaeozoic, and the Ammonitida are not only pre- 

 eminently but are 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 over a 

 thousand species have been described by M. Barrande from 

 the Silurian basin of Bohemia alone. The Nautilida proper 

 have gradually decreased in numbers from the Palaeozoic, 

 through the Secondary and Tertiary periods to the present 

 day. The Orthoceratidcz died out much sooner, being exclu- 

 sively Palaeozoic, with the exception of the genera Orthoceras 

 itself and Cyrtoceras, which survived into the commencement 

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



Fig. 114. Shells of Secondary Cephalopods. i Ancyloceras Matheronianus; 2 Sca- 

 pkites eequalis; 3 Crioceras Duvalii; 4 Hamttes attenuatus ; 5 Turrilites 

 catenatus. 



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

 nitidcz is almost exclusively Secondary, being very largely re- 

 presented by numerous species of the genera Ammonites, Cera- 



