MOLLUSC A— CEPHALOPODA. 3 1 



f. With internal deposits leaving annulated endosiphuncle 222. 



222. Orthoceracones LIX . Actinoceras. 



222. Cyrtoceracones hh. 



hh. Tube rapidly enlarging , 8f. 



Sf. Section only slightly compressed LX, Cyrtactinoceras. 



8f . Section strongly compressed ; siphuncle broad. 



LXI. Gonioceras. 



hh. Slender, slowly enlarging LXII. Mcelonoceras. 



***. Aperture slightly contracted, form elongate (longicone), slender g. 



g. Section circular or compressed dorsoventrally 333. 



333. Rapidly enlarging LXIII. Cyclostomiceras. 



333. Subcylindrical LXV. Ooceras. 



g. Laterally compressed LXIV. Oncoceras. 



***. Contraction moderate, below aperture, form slender, gently arcuate. 



LXVI. Clinoceras. 

 ***. Aperture generally strongly contracted, shell a breviconic cyrtocera- 



cone or orthoceracone h. 



h. Circular or dorsoventrally compressed 444. 



444. Aperture subtrigonal LXVIl. Poterioceras. 



444. Aperture a narrow slit in form of cross.. LXVIII. Trimeroceras. 

 444. Aperture with six lateral sinuses in addition to long hyponoraic 



sinus ; LXIX. Hexameroceras. 



444. Aperture with six lateral and one median besides hyponomic 



sinus LXX . Septameroceras. 



h. Laterally compressed LXXL Phragmoceras. 



I. Ammonoidea : Shell with strongly lobed sutures, the lobes and saddles com- 

 pound in the more specialized division — siphuncle typically ventral and mar- 

 ginal in all but one group in which it is dorsally situated ; siphuncle mostly 



with forward turning " collars " 2. 



2. Suture straight or with simple lobes and saddles (goniatitic) ****, 



****. Shells straight LXXIV. Bactritts. 



****. Shell s coiled i. 



i. Siphuncle situated dorsally or on inside of whorls 555. 



555. Discoidal, whorls but slightly enlarging, sutures scarcely lobed. 



LXXIII. Platyclymenia. 

 555- Whorls regularly enlarging, sutures with few simple but pro- 

 nounced lobes LXXII. Acanthodymenia. 



i. Siphuncle ventran or subventran 666. 



666. Ventral lobe of suture undivided ii. 



ii. With broad lateral lobe, the saddles at lateral angles. 



LXXV. Agoniatites. 



ii. With lateral lobes and saddles 9f . 



9f. Lobes and saddles nearly equal g. 



g. Minutely umbilicated LXXVI. Tornoceras. 



g. Non-umbilicate LXXIX. Parodiceras. 



9f . With broad lateral saddle and sharp lobe. 



LXXX. Aganides. 

 9f. With narrow pointed lateral lobes and generally spatu- 



late saddles XCIII. Prolecanites. 



666. Ventral lobe divided by saddle jj. 



jj. Ventral saddle notched lof . 



