ORIGIN AND CHARACTERISTICS OF SUBORDERS. 5 



saddles are phylliform. The young of Monoplvjllites Suessi, Moj., 1 of the Trias, 

 has sutures similar to the adults of Popanoceras antiquum 2 and Kingianum of the 

 Dyas, 3 which are true Goniatitinas. The sutures of Popanoceras are in their 

 turn transitional between Monophyllites and the more normal Goniatitinae of 

 the genus Prolecanites. 



Triassic Ammonoidea have shallow ventral lobes and very prominent broad 

 siphonal saddles, thus giving the first lateral saddles the aspect of being ad- 

 juncts of the siphonal saddle. In consequence of the more direct descent 

 of Lytoceratinaa of the Jura from primitive forms, their sutures persist in 

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

 saddles, with the superior laterals apparently set upon their sides, the larger 

 lobes expanded and profusely branching at the top, the saddles expanded 

 and profusely branching at the base, the auxiliary lobes and saddles more 

 numerous and more nearly equal to the larger lobes and saddles than in 

 Ammonitinaa. Neumayer has demonstrated trumpet-like apertures in Lyt. im- 

 mane* The frilled and elevated ridges in shells of many forms indicate that 

 these are perhaps not uncommon in this group. 5 



The normal forms of the Ammonitinaa, the Arietidae of the Lower Lias, 

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

 traced back to the Goniatitinaa through Arcestes of the Trias and Cyclolobus 

 of the Dyas. The Ammonitinaa do not, therefore, come directly from the 

 Goniatitinae, as do the Lytoceratinae, but are probably direct offshoots of the 

 lower Arcestinaa. The Ammonitinaa include not only the typical Jurassic and 

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

 ceras of the Lias, and Gymnites and Ptychites of the Trias. 6 



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

 compressed discoidal secondary radical, but still smooth shell, which is also 

 characteristic of Psiloceras of the Lias. The sutures are correlatively modified, 

 and begin to assume the aspect and proportions of the true Ammonitinaa. The 

 siphonal saddle is more prominent, but still retains in many species the pointed 

 aspect derived from the Goniatitinae. The narrow first lateral saddles are a]3t to 

 appear like adjuncts of the siphonal saddle, owing to the great size and breadth 



1 Mediterr. Triasprov., pi. lxxix. fig. 1 a-e. 



2 Arcestes antiquus, Waagen, Salt Range, Pal. Ind., sei\ 13, I. i, pi. i. fig. 10. 



3 Russia and Ural, M. V. K., II. pi. xxvii. fig. 5. 



4 Mojsis. et Neum., Beitr., III., 1S83, 1831, pi. xx. 



6 Schlbnbach, Paleontogr., XIII. p. 169, pi. xxvii. fig. 3, describes A mm. hhxicornis, one of the 

 Lytocerathife, having a series of prominent flaring ridges indicating permanent apertures of similar 

 form. The slight, blunt rostrum is a notable characteristic of these apertures. Unfortunately, very 

 few have been preserved, possibly owing to the fact that they were in most species, as in the two 

 mentioned, thin flaring ridges, easily destroyed. We can only suggest, therefore, that this form of 

 rostrum might have been peculiar to this suborder. 



In "Genera of Fossil Cephalopods," in 1883, we expressed this opinion as follows: "This genus 

 (Cyclolobus) is very important, since it enables us to show the gradations by which the Proleeanitidae 

 approximate to Arcestes, Ptychites, and Monophyllites." Mojsisovics, with new materials from Spits- 

 bergen, has lately demonstrated the correctness of this opinion in part, and gives conclusive evidence 

 of the probable derivation of Arcestes and Ptychites from Popanoceras. Arkt. Trias Fauna, Mem. Akad. 

 St. Petersb., XXXIII. No. 6, p. 06, pi. xv. 



