ORIGIN Wl> *ll vi: v«'i ERI9 SUBORDERS 7 



that we must reckon it among the Arietidaa. The genus Schlo- 

 theimia is also a purely Jurassic Beries, though undoubtedly triassio in respect to 

 its sutures. Th 5 Uenata i- an almost exact reproduction of 



tin- form described l>\ Mojsisovics i- ./ />' in "Jahrbuch Qeologi- 



■chen Reichsanstalt," ' ami afterwards referred to Celtitea in hi- ■• Mediterranean 

 i The pilaa cross the whorls on tin' abdomen in tin- same way, 



ami tin' general aspect of this discoidal ahell is similar. I quite likely 



thai this is ,-i young Bhell of some Bpeciea, and until it» exact affinities ran be 

 determined it is of no great value. At present it would be difficult to saj 

 with any certainty ti> what genus it might be referred. Mojsisovics was evi- 

 dontlv in doubt, rince be Btates that it may be a young form of Bome species 

 Tin- resemblance t" the young of Schlot. catenaia may !»• due 

 to a purely pathological deformation, since tin- crossing "i the abdomen by the 

 pilaa occurs from disease in many species of the Arietidaa ami other keeled i_ r n>u|i> 

 of the Jura, notwithstanding the fact that it is normal in others. These facts, 

 ami the cms of form between Schlotheimia ami Psiloceras presented by the 



- Wsehnero i by the young of this last genus, lead us t<> think that 



i- derived from Psiloceras. The Ammonitinaa of the Jura, bo 



- known, show no Bpecial traces of their prolecantian descent, except in 

 the discoidal shells ami phylliform sutures of the genera just mentioned, ami in 

 the embryonic and generalized L r <>niatitie characters >>\ the apical Btages of the 

 shell. The \entral lobe of the Ammonitinsa is deep anil narrow, the siphonal 

 saddle small but more or less by marginal lobes ami saddles. The 



lies are Inn. el ami not bo deeply divided by marginal lobes a- in 

 tin- Lytoceratinaa, the lobes an- narrower at the tops than in that suborder, ami 

 sequent!} narrower at their bases. The great Bize ami small 

 number of the lobes i- also a marked peculiarity. The superior lateral saddle* 

 and !• illy remarkable for Bize, and the auxiliary lobes anil Baddies 



much less important ami more unequal as compared with tin- lateral lobi 



nss. Tin- marginal lobes ami saddl i rule short 



ami pointed, ami the saddles rounded, but not phylliform. Possibly another 

 distinction will eventually be demonstrated in the more constricted ami rostrated 

 apertures of man} of the Ammonitinsa. The characteristics of the embryos and 

 nl the earl - do not 1\ well known to he used in 



onnection. 

 The Ammonoids, there! irding t<< are not divisible into 



rand divisions, but have »..\ suborders: the Goniatitinaa, of the Silurian. 

 1 l> and Trias; the Clymeninaa <>i' the Devonian; 



the A the D [Yias; ( of the Dj a~ ami I 



the I, F the Trias, J M ind the Amtuonitince "I 



the I ' 



Unfortunately, thi h within the i 



this monograph t<» discuss the 



