394 



NAUTILOIDEA 



position variable. Single genns, Ovthnceras (Fig. 254). — Cam- 

 l)rian to Trias. 



.FA:\r. 2. Endoceratidac. — Shell straight, siphuncle wide, mar- 

 ginal, septal necks produced into tubes fitting into one another. 

 Principal genera : Endoceras (specimens of which occur six feet 

 long), and Piloceras. — Ordovician. 



Fajl 3. Actinoceratidae. — Shell straight or slightly curved, 



siphuncle wide, contracted at 

 the septa by obstruction-rings. 

 Principal genera : Actinoceras, 

 Discosorus, Huronia, Sactoceras. 

 — Ordovician to Carboniferous. 



Fam. 4. Gom^lioeeratidae. — 

 Shell globular, straight or con- 

 siderably curved, aperture nar- 

 rowed, T-shaped, body-chamber 

 large, siphuncle varialjle in posi- 

 tion. The aperture is in some 

 cases so narrow that proljably 

 only the arms could be pro- 

 truded. Principal genus, Gom- 

 (sijsi); B, portion of the exterior of n^hoceras (Fig. 255). — Silurian. 



Orthoceras annidatiim Sowb. , x i. r a ■ i oi n 



(Woodwardian Museum, Cambridge.)' J' AM. 5. AscOCeratldae. Shell 



sac -like or flask -shaped, apex 

 truncated, unknown, body-chamber occupying nearly the whole 

 of the shell on the ventral side, contracting at the aperture, last 

 few septa coalescing on the dorsal side and encroaching upon the 

 liody chamber. The young form has a symmetrical shell like 

 Orthoceras, attached to the sac-like shell above described ; as 

 growth proceeds the former portion is thrown off. Principal 

 genera : Ascoceras, Glossoceras. — Ordovician and Silurian. 



Fam. 6. Poterioceratidae. — Shell fusiform, contracted at both 

 ends, aperture simple, siphuncle variable in position, inflated 

 between the septa. The form generally resembles Gomphoceras, 

 except for the simple aperture and fusiform shape. — Ordovician 

 to Carboniferous. 



Fam. 7. Ci/rtoceratidac— Shell conical or sub -cylindrical, 

 slightly curved, body-chamber large, siphuncle variable in posi- 

 tion. Single genus, Cyrtoceras. — Cambrian to Carboniferous. 



Fam. 8. Lituitidae. — Shell coiled in a flat, sometimes loose 



A 



Fig. 254. — A, Section of Orthoctras, 

 showing the septa (s, 5), and siphuiiulc 



