NAUTILID^. 



67 



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 



■SnjHttBfflllllUiiJLj^ 



i-Li-iii^ 



Fig. 452. — Nautilus Konincldi. Carboniferous. 



Fig. 453.— iifin'/.., ' 



Lower Siluiiau. 



number. Lastly, several Tertiary species are known, all of 

 which agree with the living Nautilus pomioilius in having 

 their surface completely smooth. 



In the genus Lituites (fig. 453) the shell is at first coiled 

 discoidally, with close or disconnected whorls ; but the last 

 chamber is produced into a straight or slightly-curved line. 

 The siphuncle is placed in the centre of the septa of the 

 shell, and the mouth of the shell is contracted and keyhole- 

 shaped. All the known species of Lituites are confined to 

 the Silurian formation ; but some occur in deposits the age 

 of which is probably Upper Cambrian. 



Ophidioceras, of the Silurian, resembles Ziticitcs, iDut the 

 terminal produced portion of the shell is short or wanting ; 

 while the Discoceras of the same formation, though resem- 

 bling the preceding in shape, possesses a simjale aperture. 



The genus Trochoceras is one which was founded by M. 

 Barrande to include certain singular Silurian Cephalopods in 

 which the shell is doubly curved. In the typical forms — 

 corresponding with Turrilites amongst the Ammonitidce — 

 the coils of the shell are in contact and pass obliquely round 

 a central axis, so that the shell becomes turreted. In other 

 cases, however, the shells are simply bent, and we have an 

 approach to the genus Cyrtoceras. 



In the genus Glymcnia (fig. 454) the shell is discoidal, 



