ORIGIN AND CHARACTERISTICS OP SUBORDERS. 7 



species shows that we must reckon it among the Arietidse. 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 JEgoceras Buonarotti in " Jahrbuch Geologi- 

 schen Reichsanstalt," J and afterwards referred to Celtites in his "Mediterranean 

 Triasprovinz." 2 The pilse 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 clue 

 to a purely pathological deformation, since the crossing of the abdomen by the 

 pilae occurs from disease in many species of the Arietidae 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 Waehneroceras, 3 and by the young of this last genus, lead us to think that 

 Schlotheimia was derived from Psiloceras. The Ammonitinoa 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 Ammonitinge 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 Lytoceratinaa, 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 Lytoceratinoa. 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 Ammonitinaa. 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 Goniatitina?, of the Silurian, 

 Devonian, Carboniferous, Dyas, and Trias ; the Clymeninse of the Devonian ; 

 the Arcestinse of the Dyas and Trias ; the Ceratitinse of the Dyas and Trias ; 

 the Lytoceratina? of the Trias, Jura, and Cretaceous ; and the Ammonitinoa 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, 



1 Vol. XIX., 1869, pi. xv. 1 p a ge 129, pi. xxix. 



3 A new genus described in this memoir. 



