Nautiloidea. 



27 



Among these nearly straight forms may also be mentioned the Cyrtoceras. 

 usually short, conical, slightly curved members of the genus GALLERY 

 Cyrtoceras (Fig. 48), which ranges from the Cambrian to the Car- 

 boniferous, the greater number of forms being found in the Silurian, case 7. 



*m. 



\ 



Fig. 48. — Cyrtoceras {Meloceras) clathratum, Foord. Sihirian : Feiiguerolles 

 (Calvados), France, a, part of shell, natural size; b, portion of surface, 

 mairnified. 



Passing to the forms which are mwe- or less discoidal, we have 

 Lituites, a genus occurring in the Ordovician rocks of North Table- 

 Germany and Sweden, in which the shell is at first coiled in one ^^^^ ' 

 plane, and then extended in a long straight piece until the aper- 

 ture is reached, the latter being contracted by lobes extending 



Ophidio- 

 ceras. 



Fig. 49. — Ophidioccras simplex^ Barrande. From tke Silurian of Bohemia. 



from its margin. Its siphuncle is tubular. Allied to LHuites, but 

 lacking the elongated straight portion, is Ophidioceras (Fig. 49), 

 known only in the Silurian rocks of England and Bohemia. 



