64 CEPHALOPODA. 



a great abundance of individuals. "With the close of the 

 Secondary epoch a large number of complex types disap- 

 peared wholly, and the order was left without any represen- 

 tative in the Tertiary rocks except the simple and ancient 

 genus Nautilus. 



As re^'ards the two Q-reat sections of the order, the Nauti- 

 lidcc are the most ancient, datino- their existence from the 

 Lower Silurian, if not from the Upper Cambrian. Not only 

 is this the case, but they are pre-eminently Palaeozoic, very 

 few generic types surviving into the Secondary period, and 

 only one into the Tertiary. The Ammonitidce, on the other 

 hand, are pre-eminently Mesozoic, and no member of this 

 group is known with certainty to have survived into the 

 Kainozoic period. This group, however, is represented by 

 two comparatively simple types in the Palaeozoic period, 

 commencing their existence from the Silurian. 



In the following are given the characters and distribution 

 in time of the leading forms of the Tetrabranchiate Cephalo- 

 pods : — 



Nautilid^. 



Fam. I. Nautilid.e. — Sutures of the shell simple; the 

 sipJmncle simple, centred, suh-cenfral, or near the concavity of 

 the curved shells. 



Eecent researches have also shown that the development 

 of the shell of the Nautilidm is effected in a manner very 

 difierent to that which obtains among the Ammonitidw. In 

 all those forms of the Nautilidm which have been examined 

 in a sufficiently early condition, or in sufficiently perfect 

 specimens, it appears that the embryonic shell or initial 

 chamber has the form of a simple cone, not in any way in- 

 flated, and not separated from the later-formed portion of the 

 shell by any constriction or distinct line of demarcation. 

 The surface of the embryonic shell is usually marked by a 

 network of transverse and longitudinal striie, which mostly 

 become obsolete in the adult shell. The extremity of the 

 initial chamljer may be pointed, but is more usually obtuse 

 and rounded (fig. 450, a and c); and it also exhibits an oval, 

 rounded, or slit-like scar or cicatrix, which marks the place 



