250 PALAEONTOLOGY. 



families, according as the siphon is in the centre, or nearly 

 in the centre, of the septum, or on the internal or external 

 border of the shell. 



The position of the siphon must, therefore, be accurately 

 determined in the study of fossil species. 



1st Family. The NAUTILID^. Siphon placed in the 

 middle, or almost in the middle, of the septa ; shell spiral or 

 straight ; septa, simple or sinuous ; never ramified, nor 

 angular at the borders; provided with an opening, generally 

 sinuous, at the internal dorsal border. 



This Family contains only one existing genus, the nautilus. 

 Shell spiral ; rolled on the same plane ; volutions at all ages 

 contiguous, apparent or concealed. This genus contains more 

 than 112 fossil species, of which the first are of the Devonian 

 stage. It had its maximum of specific development during 

 the carboniferous stage. After having traversed all the sub- 

 sequent periods, we find only two species in the warm seas 

 of the present day. They are the only representatives of the 

 once numerous and varied genera of tentaculifera. 



The NAUTILI, are almost always, during their embryonic 

 age, provided with striae, although they may be smooth in 

 later life. The species, with the volutions exposed, in general 

 belong to the palaeozoic strata. The species striated longitu- 

 dinally are generally oolitic, and the radiated species are 

 cretaceous. (Fig. 167.) 



The lituites, kortolus, nautiloceras, aploceras, gomphoceras, 

 gonioceras, orthoceratites, actinoceras, and andoceras, are 

 special to the palaeozoic rocks. 



The orthoceratites range through the palaeozoic, and extend 

 to the triasic strata.* The nautiloceras commenced in the 

 carboniferous, and became extinct at the close of the triasic 

 period, whilst the true nautilus commenced in the Devonian, 

 is found in every subsequent stage, and lives in our present 

 seas. 



2nd Family. CLTMENID^, D'Orb. Siphon placed at the 

 internal part of the septa ; shell spiral, arched, or straight ; 

 septa more or less angular at the borders. The genera of 

 this family, as melia, cameroceras, pJiragmoceras, clymenia, are 

 extinct, and special to the palaeozoic rocks. MegasipJionia is 



Pict. Atlas, PI. lix. 



