4 GENESIS OF THE ARIETIDA. 
Norites is considered by Mojsisovics as allied to Pronorites, a genus of Gonia- 
titinee, 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 Arcestinsee of the Dyas are known only by one species, described by 
Waagen, Cyelolobus Oldhami,’ which has whorls of the anarcestian shape. It 
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 dyassic 
Cyclolobus, the most ancient of the true Arcestine. If we are right, the young 
of this last form, when examined, will be found to be similar to Popanoceras 
antiquum at a stage when its sutures have not yet acquired marginal lobes. 
The siphonal saddle in these forms and in true Arcestine 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 Prolecanitide. Arcestinw, therefore, retain in their 
sutures the proportions of paleozoic forms of Goniatitinze 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 Lytoceratine form a separate phylum, distinguished usually by the 
absence of true pile (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 Lytoceratine 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 Range, 
ser. 18, I. i, pl. ii. fig. 6 
