A.MMoNoIhKA 



506 



aiiarre.-tean forms, dis.-oidal only in j.rimit ive genera. Gerontic living 

 usually more or le.-> -mit rarted laterally, beenming M.met inie- -ul.anite at tin 

 in extreme age depre-.-.-d, and trun- 

 cated or concave at tlie aperture. 

 The latter has normally in the 

 ephebic >tage a lo\\- broad ventral 

 crest, but loses this in the para- 

 gerontie sub-stage, and acquire- a 

 ventral sinus simulating that of 

 Palaeozoic and more primitive 

 forms. Saddles and loU-s eom- 

 ]>letely dividetl li\- more or less 

 complex marginals, the moiiophylli'- 



outline U'illg coiujiletely olxcui-ed 



except in the young, and in dorsal 

 sutures of -Mm- apeciea Siphonal 

 saddles long, and not very deeply 

 incised l>y marginal Io1e>. Anti- 

 si phonal lobe bifid or trifid, and 



FKJ. 11(35. 



Joannites cymbiformis, Wulfen. Upper Trias ; Raschberg, 

 near Aussee, Austria. Natural cast showing living chamber 

 (after Mojsisovics). " 



Vv:. 1166. 



Suture-line of Didijmites siibglobus, ^fojs. 

 Upper Trias ; Someraukogel, near Hall- 

 sUult (after Mojsisovics). 



complex in specialised forms. 



Other dorsals may also become quite complex, and as a 

 rule are completely 

 divided, although of 

 course simpler than the 

 external sutures. Dorsal 

 sutures in the young re- 

 semble those of Po]>'ni<>- 

 ceratiilin'. Funnels chloio- 

 choanitic in ephebji- 

 stage. 



Km. 



Claittscites tornatus, Broun sp. Ui)p-r Trias; stfinl.T-kogel, near Hallstailt, Austria. .1, /;, Siil- :ini 

 front views. C, Suturi'-lhif. 



Arcestes (Fig. 1164), Stenarcestes, Proarcestes, Pararcestes, 



Mojs. (Fig. 1165); Ithaetites, gen. nov. Type 7?. (Arc.) rhaeticua, Clark sj>. Trias. 



Family 7. Didymitidae. Mojsisovics. Conchs globose like J /,>//,/,/,, but 

 having growth-lines evenly concave on the sides instead of sigmoidal, and sutures \\ith 



