394 



NAUTILOIDEA 



chapTI 



position variable. Single genus, Orthoceras (Fig. 254). Cam- 

 brian to Trias. 



Fam. 2. Endoceratidae. — 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 Piloeeras — Ordovician. 



Fam. 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. G-omphoceratidae. — 

 Shell globular, straight or con- 

 siderably curved, aperture nar- 

 rowed, T-shaped, body-chamber 

 large, siphuncle variable in posi- 

 tion. The aperture is in some 

 cases so narrow that probably 

 only the arms could be pro- 

 Principal genus, G-om- 

 phoceras (Fig. 255). — Silurian. 

 Fam. 5. Ascoceratidae. — Shell 

 sac-like or flask-shaped, apex truncated, unknown, body-chamber 

 occupying nearly the whole of the shell on the ventral side, con- 

 tracting at the aperture, last few septa coalescing on the dorsal 

 side and encroaching upon the body-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. — Ordo- 

 vician and Silurian. 



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

 ends, aperture simple, siphuncle variable in position, inflated be-f 

 tween the septa. The form generally resembles Gomphoceras, 

 except for the simple aperture and fusiform shape. — Ordovician 

 to Carboniferous. 



Fam. 7. Cyrtoeeratidae. — 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 Orthoceras, 



showing the septa (s, .9) , and siphuncle , , 



{si, si) ; B, portion of the exterior of trUCiecl. 

 Orthoceras annulatum Sowb., X 5. 

 (Woodwardian Museum, Cambridge.) 



