ORIGIN AND CHARACTERISTICS OF SUBORDERS. vs 
species shows that we must reckon it among the Arietide. The genus Schlo- 
theimia is also a purely jurassic series, though undoubtedly triassic in respect to 
its sutures. The young of Schlotheimia catenata is an almost exact reproduction of 
the form described by Mojsisovics as @yoceras Buonarotti in “ Jahrbuch Geologi- 
schen Reichsanstalt,’' and afterwards referred to Celtites in his “ Mediterranean 
Triasprovinz.”” The pil cross the whorls on the abdomen in the same way, 
and the general aspect of this discoidal shell is similar. It seems quite likely 
that this is a young shell of some species, and until its exact affinities can be 
determined it is of no great value. At present it would be difficult to say 
with any certainty to what genus it might be referred. Mojsisovics was evi- 
dently in doubt, since he states that it may be a young form of some species 
of Balatonites. The resemblance to the young of Schlot. catenata may be due 
to a purely pathological deformation, since the crossing of the abdomen by the 
pile occurs from disease in many species of the Arietide and other keeled groups 
of the Jura, notwithstanding the fact that it is normal in others. These facts, 
and the gradations of form between Schlotheimia and Psiloceras presented by the 
genus Weehneroceras,’ and by the young of this last genus, lead us to think that 
Schlotheimia was derived from Psiloceras. The Ammonitine of the Jura, so 
far as known, show no special traces of their prolecantian descent, except in 
the discoidal shells and phylliform sutures of the genera just mentioned, and in 
the embryonic and generalized goniatitic characters of the apical stages of the 
shell. The ventral lobe of the Ammonitins is deep and narrow, the siphonal 
saddle small but more or less dentated by marginal lobes and saddles. The 
lateral saddles are broad and not so deeply divided by marginal lobes as in 
the Lytoceratinxs, the lobes are narrower at the tops than in that suborder, and 
the saddles consequently narrower at their bases. The great size and small 
number of the lobes is also a marked peculiarity. The superior lateral saddles 
and lobes are especially remarkable for size, and the auxiliary lobes and saddles 
much less important and more unequal as compared with the lateral lobes and 
saddles than in Lytoceratinee. The marginal lobes and saddles are as a rule short 
and pointed, and the saddles rounded, but not phylliform. Possibly another 
distinction will eventually be demonstrated in the more constricted and rostrated 
apertures of many of the Ammonitines. .The characteristics of the embryos and 
of the earliest stages do not yet seem sufficiently well known to be used in 
this connection. 
The Ammonoids, therefore, according to our views, are not divisible into 
two grand divisions, but have six suborders: the Goniatitinae, of the Silurian, 
Devonian, Carboniferous, Dyas, and Trias; the Clymenine of the Devonian ; 
the Arcestinss of the Dyas and Trias; the Ceratitine of the Dyas and Trias; 
the Lytoceratine of the Trias, Jura, and Cretaceous; and the Ammonitins of 
the Trias, Jura, and Cretaceous. 
Unfortunately, there is not space enough within the necessary limits of 
this monograph to discuss the classifications of Mojsisovics, Fischer, and Zittel, 
A Vol SEG, 1869, pli xy. 2 Page 129, pl. xxix. 
8 A new genus described in this memoir. 
