640 



MOLLUSCA 



PHYLUM VI 



ranRing from ceratitic U> moderately digitale. This group is probably the connecting 

 link between the Glypliioceratidae and the Haloritmae. 



J^oXrtor« Hyatt (Fig. 1210) ; HtepUnite. Waagen ; S^hntes Mojs. Tnas. 



Subfamily D. Celtitinae Mojsisovics. 



Forms discoidal, evolute and slender in most gene.a. In primitive forms with 



stron» umbüical rite, frequent constrictions and low whorls resembhng teemce,«. 



'^ Septa always consistiiig ot lew iobes 



I and saddles, ranging from goniatitic, 



; through ceratitic, to slightly digitate. 



___^ ^te Paraceltües Gemm. Permian. 



IHiiP^'V W Geltites and Tropiceltites Mojs. ; Golum- 



^^ 'i Wir hites Hyatt and Smith (Fig. 1211); 



Proteusites Hauer ; Margarites Mojs. 

 , — - ^>^ Trias. 



/ ' '^V V^ V^~V^^ This group is the connecting link 



/». '"' ■ ' \ " «^ between Gastrioceras of the Paleozoic 



and the Tropitinae of the Middle and 

 Upper Trias. 



I 



/■■ymk-^ 



,^' 



qpirrr 



Fig. 1211. 



Columhites parisianus Hyatt and Smith. Lower Trias 

 , Idaho (after Hyatt and Smith). 



Fig. 1212. 



Agathiceras ciscoense Smith. Goal 

 Measures ; Texas (after J. P. Smith). 



Family 13. Arcestidae Mojsisovics. 



Forms smooth, involute, rohust, with frequent labial constrictions. Body-chamher 

 long. Septa ranging from goniatitic, to complex digitate, hut alvjays with a tendency 

 towards the multiplication of the lateral lohes, hoth external and internal. This family 

 is undouhtedly derived from the gastrioceran hranch of the Glyphioceratidae, hut may 

 he polyphyletic, inasmuch as hoth Agathiceras and Schistoceras may he radicals of 

 suhgroups. 



Subfamily A. Pop anoce ratin ae Hyatt. 



Forms very robust and involute. Septa relatively simple, ranging from goniatitic 

 (Agathiceras), though slightly serrated (Stacheoceras), but never becoming completely 

 digitate. This is the oldest and the most primitive group of the Arcestidae, and pro- 

 bably branched out from Schistoceras, with Stacheoceras as the connecting link. 



