NAUTILOIDEA. 



843 



The species of Ascoceras are found in the Silurian rocks in Europe, 

 and in the Ordovician rocks in North America. The Silurian genus 

 Glossoceras resembles Ascoceras in general structure, but the aper- 

 ture is contracted and lobed. The name of Aphragmites, again, is 

 given to forms in which the peculiar arched septa at the side of the 

 body-chamber are absent, the shell otherwise resembling Ascoceras. 



Family 6. Poterioceratid^e. — The principal genus included in 

 this family is Poterioceras, in which the shell is fusiform in shape, 

 inflated in the central portion of its length, contracted towards the 

 aperture, and very slender in its apical portion. The siphuncle is 

 subcentral or marginal, and is inflated between the septa. The 

 shell has a general resemblance to Gomphoceras, but the aperture 

 is simple. The species of Poterioceras range from the Ordovician 

 to the Carboniferous. 



Family 7. Cyrtoceratid^e. — This family includes only the genus 

 Cyrtoceras, in which the shell (fig. 756) is more or less curved, but 



Fig. 756. — Cyrtoceras obliquum, Middle Devonian, Germany, about two-thirds of the natural 

 size. «, Front view ; b, Side view. (After Foord.) 



does not form a complete volution, and the body-chamber is of 

 large size. The septa are simple, concave forwards, and the 

 siphuncle may be internal, external, or subcentral in position, and 

 may be cylindrical or beaded. The shell is conical, or sometimes 

 subcylindrical in form, some species being of the " brevicone " type, 

 while others are "longicone." The genus Cyrtoceras ranges from 



