NAUTILID.'E. 28l 



As regards the two great sections of the order, the Nauti- 

 lidce are the most ancient, dating 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 

 forms surviving into the Secondary period, and only one into 

 the Tertiary. The Ammonitidcz, on the other hand, are pre- 

 eminently Mesozoic, and no member of this group is known 

 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^E. 



FAM. I. NAUTILID^E. Sutures of the shett simple ; thesiphunde 

 simple, central, sub-central, or near the concavity of the curved 

 shells. 



SUB-FAMILY i. NAUTILID/E PROPER. Body-chamber capa- 

 cious ; aperture of the shell simple ; siphuncle central or in- 

 ternal. The genera of this sub-family are Nautilus, Lituites, 

 Trochoceras, and Clymenia ; of which the last three are ex- 

 clusively Palaeozoic, whilst the first ranges through all the 

 great formations from the Silurian upwards, and is represented 

 at the present day by the Pearly Nautilus. 



In the genus Nautilus (fig. 250) the shell is involute or dis- 

 coidal, consisting of a few whorls coiled into a flat spiral. The 



Fig. 250. Nautilus Danicus. Upper Cretaceous (" Danien " of D'Orbigny). 



body-chamber is of large size, and the siphuncle is central, or 

 nearly so. The genus Nautilus ranges from the Upper Silu- 



