640 



MOLLUSCA 



PHYLUM VI 



ranging from ceratitic to moderately digitate. This group is probably the connecting 

 link between the Glyphioceratidae and the Haloritinae. 



Acrochordiceras RysLtt (Fig. 1210); Ste2}hanit es Waagen ; Sibirit es Mojs. Trias. 



Subfamily D. Celtitinae Mojsisovics. 



Forms discoidal, evolute and slender in most genera. In jJi'iniitive forms with 

 strong umbilical ribs, frequent constrictions and low whorls resembling Gastrioceras. 



Septa always consisting of few lobes 

 and saddles, ranging from goniatitic, 

 through ceratitic, to slightly digitate. 



Paraceltites Gemm. Permian. 

 Geltites and Tropiceliites Mojs. ; Colum- 

 bites Hyatt and Smith (Fig. 1211); 

 Proteusifes Hauer ; Margarites Mojs. 

 Trias. 



This group is the connecting link 

 between Gastrioceras of the Paleozoic 

 and the Tropitinae of the Middle and 

 Upper Trias. 



Fh,;. liill. 



Columhi.frf; pnrisiaims Hyatt and Suiith. Lowit Trias 

 Ifl.ilio (after Hyatt ami Smith). 



Fig. 1212. 



Jgiithiceras cisooense Smith. Coal 

 Measures ; Texas (after .1. P. Smith). 



Family 13. Arcestidae Mojsisovics. 



Forms smooth, involute, robust, with frequent labial constrictions. Body -chamber 

 long. Septa ranging from goniatitic, to complex digitate, but ahvays with a tendency 

 towards the nuiltiplication of the lateral lobes, both external and internal. This family 

 is undoubtedly derived from the gastrioceran branch of the Glyphioceratidae, but may 

 be polyphyletic, inasmuch as both Agathiceras and Schistoceras may be radicals of 

 subgroups. 



Subfamily A. Popanoceratinae 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. 



