AMMONOIDEA 



397 



as applied above. It is probable, however, that the position of 

 the animal of Avimonitrs in its shell resembled that of Nautilus. 

 The siphuncle is dorsal (internal) in 

 Clymeniic only, ventral (external) in all 

 other genera. 



The aptyvlmi^ of Ammonoidea is a 

 corneous or calcareous A^alve-like body, 

 generally formed of two synnnetrical 

 parts (Fig. !257). It has been regarded 

 by some as the covering of the nida- 

 mental gland, and hence as occm-ring Fig. 257.— Aptychus of Ammonite 



, . , p IT ,1 -.1 (Trigonellites latus). Kim- 



only m the lemale, by others, with more merid'^e Clay, Ely. x i. 

 probal)ility, as an operculum, covering 



or imbedded in a hood formed, as in Nautilus, of modified arms. 

 Sometimes tlie Aptychus is in a single piece {Ana^ityclnis), some- 

 times the two pieces are united on the median line {Syiuqitychus). 

 The Ammonoidea are thus classified l)y Dr. P. Fischer : — 



(rt) Eetrosiplionata .... Goniatitidae. 



I [First saddle, ( Arcestidae, Tropitidae, 



I No Aptyclius or wide \^ Ceratitidae, Chjdonitidae. 



I Anaptychus -' i Piyiacoceratidae, Amal- 



(h) Prosiplionata/ corneous, First saddle,- theidae, Ammonitidae, 



I single y narrow [ Lytoceratidae. 



I Aptyclius calcareous, valves ^ Harpoceratidae, Stephano- 

 \ double or united '( ceratidue. 



(«) Retrosiphonata. Fam. 1. Goniatitidae. — Shell nautiloid, 

 whorls sometimes disjoined, siphuncle ventral or dorsal, sutures 

 simple. Principal genera: Clymenia, Goniatites, (Fig. 258, A). 

 — Uevonian to Carboniferous. 



{h) Prusvphonata. FA:\r. 2. Arcestidae. — Shell globular, smooth 

 or striated and rayed, body-chamber very long, aperture often 

 with a projecting hood, umbilicus closed by a callosity, lobes 

 numerous, foliaceous, aptychus present. Principal genera : Arcestes, 

 Lohites. — Principally Trias. 



Fa^i. 3. Tnqntidac. — Differs from Arcestidae mainly in the 

 more highly ornamented surface, which is decorated with ril>s 

 which become granular at the periphery. Principal genus, 

 Tropitcs. — Trias and Lias. 



Fa:m. 4. Ceratitidae. — Shell ribbed and tuljerculated, body 

 chamber short, lobes denticulated, saddles simple. Principal 

 genera: Ceratites (Fig. 258, B), Trachyceras. — Principally Trias. 



