394 



NAUTILOIDEA 



CHAP. 



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 : Undoceras (specimens of which occur six feet 

 long), and Piloceras — 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 



Fig. 254. — a, Section of Orthoceras, Qj^^y ^he arms COUld be pro- 

 showing the septa (.s, .s), and siphuncle , i i -r* • • i /^ 



{si, si) ; B, portion of the exterior of trudcd. Principal gCnUS, (rOm- 



Orthoceras annuiatum Sowb., x h pJwceras (Fig. 255). — Silurian." 



( Wood wardian Museum, Cambridge.) -^„ r a ,-7 oin 



tA'M.o.Ascoceratidae. — bhell 

 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- 

 tween the septa. The form generally resembles G-ompJioceras, 

 except for the simple aperture and fusiform shape. — Ordovician 

 to Carboniferous. 



Fam. 7. Cyrtoceratidae. — 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 



