L. F. Spdth — Notes on Ammonites. 221 



suture-lines of Hamiilina and Crioceras might indicate a real relation- 

 ship "between these two genera, we have convinced ourselves that we 

 have here a simple case of convergence. The first lateral lobe, 

 which at first sight appears to he trifid in certain forms of Hamiilina, 

 is in reality a bifid lobe, deformed by the reduction of its internal 

 branch." The writer is inclined to doubt, however, whether many 

 of the apparently lytoceratid forms described as "Crioceras" by 

 Sarasin and Schondelmayer "enter into the great family of Peri- 

 sphinctidse ". 



Here the adult suture-line alone has been utilized and the result is 

 far from satisfactory, hut even where the ontogenetic development 

 has been worked out, one special peculiarity of the suture-line cannot 

 always he regarded as proof of even generic affinity. The writer has 

 shown 1 that, e.g., in Psiloceras and Tragophylloceras the external 

 saddle has throughout ontogeny a monophyllic ending, whereas in 

 Rhacophyllites stella there are two terminal leaflets. It was assumed, 

 therefore, that Tragophylloceras was more nearly related to Psiloceras 

 and what were grouped as Pleuracanthitidse 2 than to the contem- 

 poraneous Rhacophyllites. Apart from the wide separation, 

 geologically, of Tragophylloceras from the Hettangian genera, the 

 fact that not all " Rhacophyllites' " have a diphyllic external saddle, and 

 the comparative insignificance of these differences in the endings of the 

 external saddle, make itprobahlethat Tragophylloceras must be derived 

 from its Mediterranean Bhacophyllitid contemporaries. On the other 

 hand, the phylloid character of the suture-line shows that these genera 

 as well as Psiloceras are descendants of the great family Phylloceratida, 

 though few families offer such a striking characteristic of the suture- 

 line for classificatory purposes. 



The variability of the symmetrical or asymmetrical arrangement of 

 the folioles of the lobes and saddles is well illustrated in the sutural 

 development of Polymorphites ci. Jupiter given by A. E. Trueman 3 

 and also in that of Cymbites globosus, reproduced from Branco for 

 comparison. 4 In the former the external saddle of stage h is trifid 

 on one side and bifid on the opposite half of the same suture-line. 

 Similarly the first lateral lobe is unequally developed on the two 

 sides. In Cymbites globosus the first lateral lobe is bifid at first and 

 trifid afterwards, whereas the reverse is the case Avith the external 

 saddle. Also the first lateral saddle of the last stage and especially 

 the second lateral saddle are not monophyllic like the external saddle 

 of the eaz-lier stage, and therefore do not repeat the progressive 

 complication of the saddles in the manner shown by Tragophylloceras 

 Loscombi. 



But these two sutural developments are instructive from another 

 point of view. The very deep ventral lobe of Cymbites from the 

 third suture onwards shows this genus to be an Arietid, and 



1 " The Development of Tragophylloceras Loscombi " : Q.J.G.S., vol. Ixx, 

 1914. 



2 The family Pleuracanthitidee, Hyatt em. Diener, provisionally accepted in 

 the paper mentioned above, cannot be upheld. 



:i Op. cit. (Geol. Mag., 1917), p. 445, Fig. 10. 

 4 Loc. cit., p. 447, Fig. 13. 



