66 L. F. Spath — Notes on Ammonites. 



suture-line iu relation to the radius, it seems to vary with time in 

 the whole group, as has recently been shown by Lemoine (1910). 

 The same thing is noticed in Pachyceras and Tomquisies, where the 

 most recent forms have the most inverse suture-lines. We have to 

 do here, therefore, rather with a general phenomenon, appearing in 

 a parallel manner in the different branches of a given family, or 

 even in fairly distant groups, than with a special character peculiar 

 to a single branch, and permitting us, consequently, to follow it 

 through time. In this connexion it may be called to mind that in 

 quite another group of Ammonites, in Simhirskites of the Lower 

 Cretaceous, identical and exactly contemporaneous forms may have 

 either a normal or a freely inverse suture-line ($. inversus and 

 suhinversus) . It does not seem, therefore, that this character of an 

 oblique suture-line has very great importance." 



It might be thought that the character of the umbilical edge or 

 slope, and its ornament, could affect the position of the auxiliaries, 

 but the evidence in favour of this is not satisfactory. One should 

 not find in identical smooth oxycones, with an exactly similar, 

 rounded, umbilical edge, suture-lines that may be either concave 

 forwards or convex, either rising or descending towards the 

 umbilicus. But in, e.g., Cheltonia, the extremely oblique and 

 almost tangential suture-line of the sides is compensated for by 

 a very deep internal lobe, and the variation in the course of the 

 suture-line does not appear to affect the convexity of the septum as 

 a whole. It may be recalled here that the deposition of calcium- 

 carbonate, as in the recent Nautilus, probably began at the sides of 

 the shell, i.e. in the region farthest removed from the siphuncle, so 

 that dependent auxiliaries near the umbilicus suggest less penetration 

 posteriorly of the attaching fibres of the lobes. Swinnerton & 

 Trueman (p. 36) give two interesting illustrations of incomplete 

 septa in Dactylioceras and Polymorphites. They show that the septa 

 were indicated in all sutural details, and though only formed in 

 part were situated at the normal distance from the preceding septa. 



The functions of the septal edge are not impaired by the variation 

 of its course or curvature, therefore, and the comparative insignificance 

 of the obliquity is realized when the external and internal 1 suture- 

 line is considered as one whole. That in a Lioceras 3 direction and 

 curvature may vary within the same species, and that in Psiloceras* 

 even in the same individual, first a complex suture-line with 

 dependent inner lobes, and at the end a simple one with ascending 



1 The presence of a high internal saddle in certain Japanese Scaphites 

 induced Yabe ("Die Scaphiten a. d. Oberkreide von Hokkaido " : Beitr. Pal. 

 Osterr.-Ung., etc., vol. xxiii, p. 167, 1910) to create a new genus, Yezoites ; 

 but the writer would agree with Nowak (in " Untersuchungen u. Cephal. Ob. 

 Kreide in Polen", ii, Die Skaphiten : Extr. Bull. Acad. Sci. Cracovie, July, 

 1911, p. 549), who cannot admit that the internal portion of the suture-line of 

 Ammonites is the most important as regards the determination of their 

 relations, though " it must not be underestimated or, still less, neglected, as is 

 still done very often at the present day ' ' . 



2 Horn, " Die Harpoceraten der Murchisonae-Schichten des Donau-Khein- 

 Zuges" : Mitt. Grossh. Bad. Geol. Land. Anst., vol. vi, pt. i, p. 264. 



3 Neumayr, " Kenntn. Fauna Unterst. Lias i. Nordalpen " : Abh. k.k. 

 Beichsanst., vol. vii, pt. v, p. 25, pi. iv, figs. 6a, b, 1879. 



