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IV.—Notes on Ammonites

Published online by Cambridge University Press:  01 May 2009

L. F. Spath
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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.

Type
Original Articles
Information
Geological Magazine , Volume 6 , Issue 4 , April 1919 , pp. 170 - 177
Copyright
Copyright © Cambridge University Press 1919

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References

page 170 note 1 The borehole at Sheraton appears also to have entered the Ganister Series. Thick beds of sandstone have been penetrated dipping at an angle of 45°, probably northwards, as shown in section.

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page 170 note 7 Definitely given as “capricornum zone” in Mr.Buckman, 's “Palæontological Classification, etc.”, in The Geology of the Country between Whitby and Scarborough (Mem. Geol. Surv.), 2nd ed., 1915Google Scholar.

page 171 note 1 Monograph of the Lias Ammonites, Pal. Soc., 1880, p. 219Google Scholar. Only seven years after the compilation of Wright's work, hailed at the time of its appearance as a “masterly monograph” (Geikie, A., Text-Book of Geology, 1882, p. 786Google Scholar), ProfessorBlake, (“The Evolution and Classification of the Cephalopoda, etc”: Proc. Geol. Assoc., vol. xii, p. 292, 1892)Google Scholar had to say with regard to the classification adopted by Wright, namely that of Neumayr, originally published in 1875: “Its author, were he happily still with us, would certainly regard it as quite inadequate and out of date at the present time.”

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page 174 note 4 The writer used this character in the subdivision of the Middle Liassic (Domerian) Hildoceratids (“On Jurassic Ammonites from Jebel Zaghuan”: Q.J.G.S., vol. lxix, pp. 547–52, 1913Google Scholar) and separated the Flexiradiata from the rectiradiate forms that constitute the genus Seguenziceras. The genus Protogrammoceras was created for the former, and two divisions were recognized within that genus; but one of these, characterized by dionase in peripheral projection and including subanguliradiate and angulirursiradiate forms, is covered by the genus Fuciniceras created just prior to the publication of the writer's paper. The genus Protogrammoceras will, therefore, have to be restricted to the forms of the first subdivision, including subfalciradiate and falciradiate forms (type “Grammoceras” bassanii, Fucini, “Apenn. Centr.,” pl. x, fig. 6, 1900). Apart from their Domerian age, both the Rursiradiata (Fuciniceras) and the Falciradiata (Protogrammoceras) are distinguished from the Toarcian Harpocerates by their combination of evolute whorls with a tendency to change the periphery from fastigate to carinatisulcate and back again to fastigate. The form described and figured in that paper as gen. nov. sp. nov.(?) (pl. lii, fig. 2, p. 556) belongs to the group of forms wrongly referred to Harpoceratoides by Haas, and the new genus Lioceratoides (type “Lioceras (?)” Grecoi, Fucini, , “Apenn. Centr.,” Pal. Ital., vol. vi, p. 65, pl. xi, fig. 4, 1900Google Scholar) is now proposed for this development, characterized by a type of costation very distinct from that of the other Domerian Hildoceratids.

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