398 



AMMONOIDEA 



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

 short, sutural line undulated, simple. Principal genera: Chjdonites, 

 Choristoceras, Hhahdoceras, Cochloceras. — Trias. 



Fam. 6. Piaacoceratidae. — Shell discoidal, usually smooth, 

 liody-chamber short, sutural line very complex, lobes numerous. 

 Principal genera : Finacoceras, Sageceras. — Carboniferous to Trias. 



Fam. 7. AmaWieidae. — Shell broad, keeled, last whorl con- 

 cealing most of the spire, sutures with auxiliary lobes, incised. 

 — -Principal genera: AmaWicvs, Sehlocnhacta, Sphenodiscus. — 

 Trias, Cretaceous. 



Fam. 8. Amiiwiiitidac. — Body-chamber long, whorls narrow, 



Fig. 258. — Various forms of Ammonoidea : A, G'oniatites crenistria J. Phil., Carb. 

 Limestone ; B, Ceratites nodosiis de Hanii., IMusclielkalk ; C, Ammonites (Parkin- 

 sonia) Parkinsoni Sowh. , Inf. Oolite; D, Plrjlloceras hete/vjj/iijllum Sowh., Ujiper 

 Lias ; s, s, sutural lines. 



uncovered, more or less ribbed, aperture simple, sutural line 

 normal, aptychus single, corneous. Principal genera : Ammonites, 

 Aegoceras. — Principally Lias. 



Fam. 9. Lytoceratidae, — Shell discoidal, body-chamber short, 

 aperture simple, no aptychus. Principal genera : Lytoceras, 

 PhylloceTas (Fig. 258, D). — Trias to Cretaceous. 



Fam. 10. Haiyoceratidae. — Shell discoidal, compressed, 

 margin keeled, surface with straight or arched ribs, aperture 



