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The variability and occasional instability of the Ammonoid suture-line, to which attention has been drawn, the recurrence of similar types, and the frequent asymmetry of the opposing halves of a given suture-line, which is apparent not only in the Dactylioceras commune, figured by Swinnerton & Trueman (fig. 9 on p. 42), but also in the development of the suture-line in e.g. Pseudosageceras multilobatum, Noetling, in Indoceras baluchistanense, Noetling, or in Oxynoticeras oxynotum, Quenstedt, sp., to mention only a few well-illustrated examples, might be thought to impair the usefulness of the suture-line for the classification of Ammonoids. Yet, long before there was any subdivision of “Ammonites” at all, the greatest importance had been attached to the foldings of the suture-line, and Pictet stated in 1854 that “the lobes in their essential traits furnished very constant and very valuable characters”. Von Buch's group of “Arietes” was well characterized by the general plan of the suture-line, namely, the deep siphonal lobe and the short external saddle, only most authors would put more reliance on the ornamentation of the shell and put such a form as Asteroceras sagittarium, Blake, sp., into the genus “Aegoceras.” The writer would even go so far as to say that the type of suture-line given by Mr. Buckman for “Defossiceras” defossum, Simpson, sp., should not be found at the horizon stated, and that the form probably will turn out to be an Arietid (Agassiceras) of semicostatum age.
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