352 me. l. f. spatii ox the [Dec. 1 914, 



descendant of Mojsvarites clio, as Mojsisovics had stated (1893), 

 nor of M. planorhoides, as Prof. Haug (1908) seems to think; 

 for this genns had attained already a fairly involute condition in 

 the Rhsetic, whereas Psiloceras (and especially Parapsiloceras, 

 which resembles so closely the primitive Pleuracanthites polycy- 

 cloides TV aim.) is still latumbilicate. It is neither a derivative of 

 the diphyllic DiscojihyUites nor of Phylloceras, as Pompeckj 

 (1895, p. 40) thinks ; but comes from a branch of the original 

 Monophyllites root-stock that may have given rise to Mojsvarites 

 as well, although it persisted itself in a less advanced condition 

 through Rhsetic times, having possibly not only dependent 

 auxiliaries, but also the ornament of the inner whorls of certain 

 forms of Monophyllites. Dr. Perrin-Smith (1913) thinks it im- 

 probable that Psiloceras can be derived from Phylloceratidse, and 

 states that the resemblance to Monophyllites probably is only 

 a convergence-phenomenon. He considers it ' just as likely ' that 

 Psiloceras is a degenerate series reversionary towards the ancestral 

 radical ; but these are mere conjectures. Mr. Buckman, however, 

 has stated as recently as 1912 (op. cit. p. vii) that Psiloceras is a 

 ' degenerate, (smooth) derivative of a Caloceras stock ' ; but this 

 cannot be admitted in view of the phylloid suture-line. The 

 development of the suture-line shows that that of the later Calo- 

 ceras results normally from that of the earlier Psiloceras, as a 

 further modification of the original Moj svaritesASkc suture. The 

 similarity of the suture-development to that of Tragopliylloceras 

 Iosco mb i and, less so, to the suture-development of Phylloceras 

 heterophyllum, shows that the ancestor of Psiloceras must be 

 looked for in the Monophyllitidse. 



The absence of a suspensive lobe (dependent auxiliaries) in forms 

 of the latter family is, in my opinion, not of great importance. 

 "Wanner (1886) has already pointed out that it is neither always 

 present in Psiloceras, nor always absent in ' Ariel if esS Its 

 significance is doubtful ; for it is found in evolute and in involute 

 shells, in ornamented and in smooth forms, in ammonites with few 

 septa as in those with crowded sutures. Some smooth cata genetic 

 oxycones show a strong forward inclination of the auxiliaries, others 

 have dependent inner lobes. On the other hand, Wcelineroceras, 

 admittedly a costate Psiloceras development, shows the dependent 

 inner lobes ; whereas the even more anagenetic (in costation) 

 ' Psiloceras ' capra-ihex Pompeckj, from the JBuchlandi Zone, has 

 a strong forward inclination of the auxiliaries. 1 Thus, in view of 

 the evidence of the phylloid suture, Ave are not justified in laying 



1 It may not be out of place to mention here that, although some Arietoids 

 seem to be developments of Caloceras and Alsatites, yet others show onto- 

 genetic characters which do not suggest a derivation from Psiioceratida3. For 

 example, Vermiceras spiratissimum (Quenstedt) has not only a persistently 

 low ventral lobe, but the external saddle is still larger than the first lateral 

 saddle at a diameter of 8 mm. ; and, at a diameter of 4 mm., the suture is 

 at least as simple as it is in the Triassic Monophyllites svmonyi and in 

 Psiloceras aff. erugatum at 3 and 2 - 25 mm. respectively. 



