MOLLUSCA— CEPHALOPODA. 35 



1. Whorls depressed, form subglobose 2222. 



2222. Venter with keel ; umbilicus profound with margin noded. 



CXIII. Tropites.. 



2222. Venter without keel ; umbilicus small CXVI. Arcestes. 



6*. Saddles deeply incised by secondary lobes (except in retarded species). .m. 



m. Whorls coiling in single plane 3333' 



3333. Close coiled throughout, with rounded venter uu. 



uu. Surface without ribs, but often with varices 22f . 



22f . Secondary saddles rounded, leaf-like, narrowed at base..(?. 



0. Whorls strongly compressed, umbilicus small or absent. 



CXIX. Phylloceras. 



0. Whorls vertically depressed, umbilicus of moderate size. 



XCI. Waagenoceras. 



22f. Secondary saddles not leaf-like ./. 



p. Whorls broadly rounded or subangular 9'. 



9'. Umbilicus rather small, surface smooth, varices or 



constrictions absent or few h''. 



h^. Growth lines falcate, outlining lateral and ven- 

 tral lappets CXXIII. Haploceras. 



W . Growth lines scarcely falcate. 



• CXXII. Pseudophyllites. 



</. Varices or constrictions present. Umbilicus often 



closed by callus CXVII. Joannites. 



i>. Umbilicus large, varices or constrictions common... ic/. 



10''. Section trapezoidal CXX. Tetragonites. 



lO''. Section rounded, oblong, moderately embracing. 



V . 

 V . Young whorls trapezoidal. 



CXXVIII. Gabbioceras. 

 V , Young whorls not trapezoidal. 



CXXI. Gaudryceras. 

 \o\ Section elongate with flat or compressed sides. 



CXXIX. Pletiropachy discus. 



lO''. Section nearly circular, whorls scarcely impressed. 



CXX XIII. Lytoceras. 



uu. Surface with ribs more or less well developed 23t. 



23|. Ribs simple (rarely bifurcating near umbilicus), increas- 

 ing by intercalation q. 



q. Ribs smooth, without tubercles or nodes \i' . 



\\' . Ribs flexed in sigmoid curves, bending forward 



across venter CXXV. Puzozia. 



II''. Ribs straight or simply curved; thin, distant, 



often obsolete in adult CXXX. Pachydiscus. 



II''. Ribs forming coarse folds on venter, obsolete 



towards umbilicus CXXIV. Eurynoticeras. 



q. Ribs more or less strongly noded 12^. 



12-^. Ribs extending across venter y. 



V . Ribs bifurcating from umbilical tubercle. 



CXXX. Pachydiscus^ 



y . Ribs simple or bifurcating tubercles on side and 



on venter CL. Acanthoceras . 



