

358 me. l. f. spath ox the [Dec. 1 9 14, 



with incipient carination, brings out the transition to Lytoceras, 

 Psiloceras, and the Arietoicls, but not the phylloceratid 

 ancestry of the group. The term Pleurae an t hit id a?, however, 

 is here adopted, and it is suggested that the family be included 

 as Monophyllit'es-Mojsvariies descendants in the superfamily 

 Phylloceratida. 



Finally, it may not be out of place to review the evidence 

 regarding pre-Liassic ancestors of Tragophylloceras and the Phyl- 

 loceratida. The sutures representing the 'Primitive Groniatite/ the 

 ' Glyphioceras-Dimorphoceras^ and the l Monophyllites^ stages 

 are again illustrated in text-figs. 3 a— 3 c (p. 357). Figs, e-li show 

 the closely-comparable development of the suture of Monophyllites 

 simonyi, after Branco. It will be seen that the suture of an adult 

 Anthracoceras {JDiniorphoceras), shown in fig. i, differs only in the 

 complication of the median saddle in the ventral lobe, a feature 

 which characterizes the development Dimorphoceras— Thalassoceras 

 (figs. k-n). Thalassocerata of the complicated genwiellaroi type, 

 however, form a specialized lateral branch, and have no descendants 

 in the Lower Trias ; whereas from simple, more generalized and 

 therefore plastic Thalassocerata (see figs, o-r), both Ussuria and 

 Ussurites appear to have developed. Here the elaboration of the 

 lateral elements takes the place of the complication of the ventral 

 lobe in the older sequence Dimorphocer as— Thalassoceras. Now, in 

 Monophyllites itself, which is shown by its suture to be nearly 

 related to the ancestral stock that produced Ussurites, the first 

 lateral lobe becomes subdivided before the ventral lobe, and this is 

 also the case in Tragophylloceras ; but it does not apply to all 

 Phylloceratida, as the ontogeny of Pltylloceras heterophyllutn 

 shows. 



Again, we have seen that the ventral lobe of Tragophylloceras 

 is considerably less deep than it is in Monopliyllites, which is what 

 we should expect in the later genus, considering the high ventral 

 lobe of the adult Monophyllites. But, arguing phylogenetically 

 from this, we should in the ancestral forms of this genus look for a 

 deep ventral lobe. It is therefore probable that a primitive and 

 originally (not secondarily) involute ammonoidof the Thalassoceras 

 microdiscus pattern, with a deep ventral lobe, and itself the de- 

 scendant of an involute glyphioceratid (Anthracoceras^}, comes 

 nearest the hypothetical Monophyllites ancestor. This generalized 

 root-form persisted throughout, and. after one or two more or less 

 futile attempts at specialization (resulting in Thalassoceras, the 

 highest-developed ammonoid of the Permian, and in Ussuria- 



1 Prof. Freeh (1899) derives Anthracoceras and IHmorphoceras from the 

 evolute Devonian Gephyroceras,\m\, it can much more naturally be attached to 

 the involute Carboniferous Ghjpfiioceras (Bey rid toreros), which has a similar 

 suture and the subdivision of the median saddle in the ventral lobe just 

 indicated, though in Dimorplwceras this subdivision becomes the prominent 

 feature. Dr. Perrin-Smith (1913, p. 637), on the other hand, connects Di- 

 morphoceras with Agon ides, but the narrow, deep, and undivided ventral lobe 

 of the latter genus is against this derivation. 



