AMMONOIDEA 



399 



with lateral projections, suture with accessory lobes, aptychus iu 

 two pieces. Principal genera : Har2)oc€ras, Opi)elia, Lissoceras. 

 —Jurassic to Cretaceous. 



Fam. 11. Stcphanoceratidae. — Shell discoidal, lielicoid or 

 straight, whorls sometimes disunited, surface often with bifur- 

 cating ribs, whicli are tuljercled, 

 aperture often with lateral pro- 

 jections, sutural line incised, 

 aptychus in two pieces, some- 

 times iniited. 



In the discoidal group, Stc- 

 'jyhanoceras is strongly ribbed, 

 tubercled at the point of bifur- 

 cation, Cusmoceras has long- 

 lateral projections of the aper- 

 ture when young, Pcrispliinctcs 

 has a large body chamber and 

 numerous smooth ribs. Other 

 genera are Acanthoceras, Pdto- 

 ceras, Asjndoccras, and Hoplites. 

 Among the loosely whorled 

 genera, Scaphitcs (Fig. 260, A) 

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

 form, while the early whorls are concealed; Hamitcs, Hainalina, 

 and Ptychoccras have a shell shaped like a. single or douljle hook, 



the sides of which 

 may or may not be 

 luiited ; Crioceras 

 (Fig. 260, B) in form 

 of whorls resembles a 

 Sjnrula, Ancyloceras 

 a Scccphites with 

 the first whorls dis- 

 united. Macrosca- 

 jJtites (Fig. 259, B) 

 is similar, but with 

 FifJ. 2G0. — A, Scapkiles aequalis Sowb., Cret.aceou.s ; B, the lirst whorls 



B 



Fig. 259. — A, Turrilites catenulatus d'Orb, 

 Gault ; B, Macroscaphites Iranil d'Orb, 

 Upper Neocomiaii. (From Zittel.) 



united and not cou- 



Vriocerus hifurcatam Qiieust., Cretaceous. (From Zittel.) 



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

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



