66 



CEPHALOPODA. 



the views of Barrande, the ovisac of the Aimnonitidm is a 

 structure not represented at all in the embryonic shell of the 

 Nautilidce. 



Sub-family 1. Nautilid^. Proper. — Body-chamlier capa- 

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

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

 Lituites, Trochoccras, and Clymenia, of which the last three 

 are exclusively Paheozoic, whilst the first ranges through all 

 the great formations from the Silurian upwards, and is repre- 

 sented at the present day by the Pearly Nautilus. 



In the genus Nautilus (fig. 451) the shell is involute or 

 discoidal, consisting of a few whorls coiled into a flat spiral. 



Fig. i^\.— Nautilus Danims. Upper Cretaceous (" Daiiieii " of D'Orbigny). 



The body-chamber is of large size, and the siphuncle is cen- 

 tral, or nearly so. The genus Nautilus ranges from the Upper 

 Silurian to the present day, having its maximum of develop- 

 ment in the Carboniferous period. The Palaeozoic forms (fig. 

 452) are mostly discoidal, having the whorls more or less 

 completely exposed. The Nautili of later deposits are mostly 

 like the living species in having each whorl overlapping the 

 preceding, so that merely an " umbilicus " is visible. Many 

 of the extinct forms, belonging to all ages, agree with the 

 living Nautilus in having the surface quite smooth {Loivigati). 

 Others, which are especially characteristic of the Jurassic 

 rocks, have the surface striated (Striati). Others, chiefly 

 of Cretaceous age, have the surface marked by distinct ribs 

 (Badiati). 



In the Upper Silurian and Devonian rocks Nautili are 



