XIII AMMONOIDEA 397 



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

 The siphuncle is dorsal (internal) in 

 Clymenia only, ventral (external) in 

 all other genera. 



The aptychus of Ammonoidea is a 

 corneous or calcareous valve-like body, 

 generally formed of two symmetrical 

 parts (Fig. 257) . It has been regarded 

 by some as the covering of the nida- 

 mental gland, and hence as occurring Fig. 257.— Aptychus of Ammonite 

 only in the female, by others, with more i^erMgectay! ^y.^^'x ^ ^'"'" 

 probability, as an operculum, covering 



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

 Sometimes the Aptychus is in a single piece QAnaptychus^^ some- 

 times the two pieces are united on the median line {Syna'ptychus), 



The Ammonoidea are thus classified by Dr. P. Fischer : — 



(a) Retrosiphonata Goniatitidae. 



' First saddle, [ Arcestidae, Tropitidae, 



No Aptychus or wide [ Ceratitidae, Clydonitidae. 



Anaptychus - ( Pinacoceratidae, Amal- 



(b) Prosiphonata j corneous, First saddle, j theidae, Ammonitidae, 

 single 1^ narrow I Lytoceratidae . 



Aptychus calcareous, valves J Harpoceratidae, Stephano- 

 double or united 1^ ceratidae. 



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

 whorls sometimes disjoined, siphuncle ventral or dorsal, sutures 

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

 — Devonian to Carboniferous. 



(5) Frosiphonata. Fam. 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 gen- 

 era : Arcestes^ Lohites. — Principally Trias. 



Fam. 3. Tropitidae. — Differs from Arcestidae mainly in the 

 more highly ornamented surface, which is decorated with ribs 

 which become granular at the periphery. Principal genus, 

 Tropites. — Trias and Lias. 



Fam. 4. Ceratitidae. — Shell ribbed and tuberculated, body- 

 chamber short, lobes denticulated, saddles simple. Principal 

 genera: Oeratites (Fig. 258, B), Trachyeeras. — Principally Trias. 



Fam. 5. Clydonitidae. — Shell variable in form, body-chamber 



