XIII 



AMMONOIDEA 



399 



with lateral projections, suture with accessory lobes, aptychus in 

 two pieces. Principal genera: Harpoceras^ Oppelia^ Lissoceras, 

 — Jurassic to Cretaceous. 



Fam. 11. Stephanoceratidae. — Shell discoidal, helicoid or 

 straight, whorls sometimes disunited, surface often with bifur- 

 cating ribs, which are tubercled, 



aperture often with lateral pro- 

 jections, sutural line incised, 

 aptychus in two pieces, some- 

 times united. 



In the discoidal group, Ste- 

 phanoceras is strongly ribbed, 

 tubercled at the point of bifur- 

 cation, Cosmoceras has long 

 lateral projections of the aper- 

 ture when young, Perisphinctes 

 has a large bodj^-chamber and 

 numerous smooth ribs. Other 

 genera are Acantlioceras^ Pelto- 



CeraS, AspidoceraS, and RopliteS. ^^^ 059.- A, TarrUites catenulatus d'Orb, 



Gault; B, MacroscaphitesIraniid'Orh, 

 Upper Neocomian. (From Zittel.) 



Among the loosely whorled 

 genera, Seaphites (Fig. 260, A) 

 has the last whorl produced and bent back again in horse-shoe 

 form, while the early whorls are concealed ; Hamites, Hamulina^ 

 and Ptychoceras have a shell shaped like a single or double hook, 



the sides of which 

 may or may not be 

 united ; Crioceras 

 (Fig. 260, B) in form 

 of whorls resembles 

 a Spirula, Ancylo- 

 ceras a Seaphites with 

 the first whorls dis- 

 united. Macrosca- 

 phites (Fig. 259, B) 

 ^ B is similar, but with 



Fig. 260. — A, Seaphites aequalis Sowh., CretSLceons; B, the first whorls 

 Crioceras 6(/Mrca^M?n Quenst., Cretaceous. (From Zittel.) "f /I A 4- 



cealed. Turrilites (Fig. 259, A) is turreted and sinistral, while 

 Baculites is quite straight, with a long body-chamber. 



