4 GENESIS OF THE ARIETID.E. 



Norites is considered by Mojsisovics as allied to Pronorites, a genus of Gonia- 

 titinre, and by Griesbach, Zittel, and the author as allied more nearly to 

 another genus of the same suborder, Sageceras. Throughout the group the 

 lobes and saddles form a simple series in which very little differentiation is 

 observable except in the highest forms. The ventral lobe is very broad and 

 short, and the siphonal saddle broad and shallow. The survival of prolecanitian 

 characters in these outlines is apparent the moment we dispense with the 

 denticulations of the lobes and reduce the sutures to their primitive outlines. 

 The Arcestina? of the Dyas are known only by one species, described by 

 Waagen, Cycblobus Oldhami, 1 which has whorls of the anarcestian shape, ft 

 is an involute species, and there may be others of this genus in the same 

 formation, not yet discovered, which have more discoidal whorls. 



According to our mode of translating the affinities of the forms, they arrange 

 themselves as follows. Popanoceras of the Dyas, as the direct descendant of 

 Prolecanites, inherits the tendency to have lobes and saddles of very nearly 

 the same size, with lobes having trifid or bifid terminations similar to those of 

 the young of Monophyllites, and also transitional to the sutures of the clyassic 

 Cyclolobus, the most ancient of the true Arcestinae. If we are right, the young 

 of this last form, when examined, will be found to be similar to Pc^xinoceras 

 antiquum at a stage when its sutures have not yet acquired marginal lobes. 



The siphonal saddle in these forms and in true Arcestinaa is small, often 

 attenuated, and the ventral lobe large and often broad. The remaining 

 lobes and saddles are more nearly of the same size, numerous, and formed 

 a gradually lessening series inclining towards the umbilicus. The same aspect 

 is common in the simpler shells of Megaphyllites and Monophyllites, but in 

 these the large phylliform saddles, with entire outlines at their bases, exhibit 

 closer approach to the Prolecanitidse. Arcestinaa, therefore, retain in their 

 sutures the proportions of paleozoic forms of Goniatitinas which have numerous 

 lobes, but depart from them in having more complicated and ornate marginal 

 digitations. The series, with some exceptions, have involved whorls which 

 can only be considered as parallel with the more involute shells of silurian 

 and devonian Anarcestes. With respect to its forms and the smoothness of the 

 shell this series is a survival of purely paleozoic modifications. 



The Lytoceratinoe form a separate phylum, distinguished usually by the 

 absence of true piloe (ribs), the larval form and characteristics of the adult 

 shell, and the leaf-shaped marginal saddles of the sutures. Lytoceras, in its 

 smooth or unpilated shell, rounded abdomen, peculiar siphonal saddle, and 

 phylliform marginal saddles, appears to be a more progressive form of the 

 same genetic series as Megaphyllites and Monophyllites of the Trias. Even 

 the peculiar coarse striations of the shells of these genera are often repeated 

 among the Lytoceratinaa of the Jura. 



Megaphyllites of the Trias is evidently closely allied to Monophyllites. 

 The siphonal saddle is similar to that of Monophyllites, and the marginal 



1 Arcestes priscus, Waagen, is probably also a species of Cyclolobus. Geol. Surv Ind., Salt Ran^e 

 ser 13. I. i. pi. ii. tig. G. 



