ORDER II AMMONOIDEA 565 
anarcestean forms, discoidal only in primitive genera. Gerontic living chamber 
usually more or less contracted laterally, becoming sometimes subacute at the venter ; 
in extreme age depressed, and trun- 
cated or concave at the aperture. 
The latter has normally in the 
ephebic stage a low broad ventral 
crest, but loses this in the para- 
gerontic sub-stage, and acquires a 
ventral sinus simulating that of 
Palaeozoic and more primitive 
forms. Saddles and lobes com- 
pletely divided by more or less 
complex marginals, the monophyllic 
outline being completely obscured 
except in the young, and in dorsal 
sutures of some species. Siphonal 
saddles long, and not very deeply 
incised by marginal lobes. Anti- 
siphonal lobe bifid or trifid, and 

Fic. 1166. Fic. 1165. 
Suture-line of Didymites subglobus, Mojs. Joannites cymbiformis, Wulfen. Upper Trias; Raschberg, 
Upper Trias; Someraukogel, near Hall- near Aussee, Austria. Natural cast showing living chamber 
stadt (after Mojsisovics). (after Mojsisoyics). 
complex in specialised forms. Other dorsals may also become quite complex, and as a 
; rule are completely 
divided, although of 
=. a7 \a\ course simpler than the 
external sutures. Dorsal 
sutures in the young re- 
semble those of Popano- 
ceratidae. Funnels chloio- 
choanitic in ephebie 
stage. 
Isculites, Sphingttes, 

Cladiscites tornatus, Bronn sp. Upper Trias; Steinbergkogel, near Hallstadt, Austria. 4, B, Side and 
front views. C, Suture-line. 

Arcestes (Fig. 1164), Stenarcestes, Proarcestes, Pararcestes, Ptycharcestes, Joannites, 
Mojs. (Fig. 1165); Rhaetites, gen. nov. Type R. (Are) rhaeticus, Clark sp. Trias. 
Family 7. Didymitidae. Mojsisovics. Conchs globose like Arcestidae, but 
having growth-lines evenly coneave on the sides instead of sigmoidal, and sutures with 
