282 



MOLLUSCA. 



Fig. 251. Lituites cornu-arietis. 

 Lower Silurian. 



rian to the present day, having its maximum of development 

 in the Carboniferous period. The Palaeozoic forms are mostly 



discoidal, having the whorls more 

 or less completely exposed. The 

 Nautili Q{ later deposits are mostly 

 like the living species in having 

 each whorl overlapping the pre- 

 ceding, so that merely an " um- 

 bilicus " is visible. Many of the 

 extinct forms, belonging to all 

 ages, agree with the living Naut- 

 ilus in having the surface quite 

 smooth ( Lcevigati ). Others, 

 which are especially character- 

 istic of the Jurassic Rocks, have 

 the surface striated (Striati). 

 Others, chiefly of Cretaceous 

 age, have the surface marked by distinct ribs (Radiati). 



In the Upper Silurian and Devonian Rocks Nautili are few; 

 in the Carboniferous, many species are known; in the Permian 



Rocks and Trias are 

 but few species; but the 

 Jurassic and Cretaceous 

 Rocks have yielded a 

 considerable number. 

 Lastly, several Tertiary 

 species are known, all 

 of which agree with the 

 living Nautilus pompilius 

 in having their surface 

 completely smooth. 



In the genus Lituites 

 (fig. 251) the shell is at 

 first coiled discoidally 

 with close or discon- 

 nected whorls ; but the 

 last chamber is pro- 

 duced into a straight 

 or slightly-curved line. 

 The siphuncle is placed 

 in the centre of the septa 

 of the shell. All the 

 known species of Lituites are confined to the Silurian forma- 

 tion ; but some occur in deposits the age of which is pro- 

 bably Upper Cambrian. 



Fig. 252. Clyinenia Sedgwickii. Devonian. The 

 lower figure shows the form of the suture. 



