ORIGIN ANI> CHARACTERISTICS OF SUBORDERS. 



saddles are phylliform. The young of '/ - . Moj., 1 of the Trias, 



ihur tn the adults ol P ntiqvvm* and Ktagituum of the 



which are true Goniatitinss. The sutures of Popanoceras are in their 



turn transitional between Monophyllitea and the more normal Goniatitinsa c ■ t 



the genus Proleoanitea 



Ammonoidea have shallow ventral lobes and very prominent broad 



riphonal Baddies, thus giving the first lateral Baddies the aspect of being 



iuncts of the riphonal saddle. In consequent <>\ the more direct descent 



of LytoceratinsB of the Jura from primitive forma, their Butures persist in 



ng triassic outlines, having usually short abdominal lobes, large siphonal 



aaddles, with the superior laterals apparently Bet upon their Bides, the larger 



tided and profusely branching at the top, the Baddies expanded 



and profusely branching at th<- base, the auxiliary lobes and Baddies more 



numerous and more nearly equal t<> the larger lobes and Baddies than in 



Ammonitin N tnayer bas demonstrated trumpet-like apertures in /,///. m- 



The frilled and elevated ridges in Bhells of many forms indicate that 



terhaps not uncommon in this group.' 



The Dormal forms of the Ammonitinae, the Arietidse of the Lower I 



• united to the genus Gymnites through Psiloceras. Gymnites can be 



traced back t<> the Goniatitinsa through Arcestes of the Trias and Cyclolobus 



of the I' - The AmmonitinoB do not, therefore, come directly from the 



Goniatitime, as do the Kyt> ratinsB, but are probably direct offshoots of the 



The AmmonitinsB include not only the typical Jurassic and 



forms, but also the allied radical genera Schlotheimia and Psilo- 



I. is, and Gymnites and Ptychites of the Triai 



In Gymnites of the Trias, the primary radical is exchanged for the more 



compressed di* mdary radical, but -till smooth shell, which is also 



! The sutures are correlatively modified, 



and b uume the as| t and proportions of the true Ammonitinro. The 



siphonal saddle is more prominent, but >till retain- in many Bpecies the pointed 



veil from the Linn Hie narrow first lateral -addle- are apt to 



appear lik< ibonal raddle, owing to the great -i/e ami breadth 



■ 



K . 1 1 i I 



■ f tli» 



f*w ha 



I 



I 



i 

 WXIII S 



