THE MOEPHOLOGT OF * STEPHANOCERAS ' ZIGZAG. 447 



29. The Morphology of * Stephais^oceras ' zigzag. By S. S. 

 EtJCKMAN, Esq., P.G.S. (Eead May 25th, 1892.) 



[Plates XIII. & XIV.] 

 Il^TRODUCTION. 



Man^y years must elapse ere I shall be able to describe Steplianoceras 

 zigzag in the volumes of the Palseontographical Society. Meanwhile , 

 the material which has come into my possession during the last few 

 years throws an important light on the developments of this fossil ; 

 and sach developments seem to supply a missing link in the connexion 

 of Balhonian and Bajocian species. jN"ot only is Steplianoceras zigzag 

 raised from comparative obscurity to the position of probable pro- 

 genitor of a large series of Bathonian species, but it is more than 

 likely that the evidence now brought forward will be of considerable 

 service to those engaged in studying the genealogy of the later species. 

 For these reasons I submit the following paper to the Society. 



D'Orbigny's figures ^ of Ammonites zigzag represent only small 

 specimens of two varieties of the species. Quenstedt figures ^ as 

 euryodos a more quickly-coiled, more coarsely-ribbed form ; but this 

 is also small. He says it is the same as d'Orbigny's zigzag. 



In a former paper on Inferior Oolite Ammonites,^ I remarked on the 

 change of form which appears to have escaped previous notice ; but 

 the specimens then referred to were only a little larger than 

 the Continental examples. The best figures of the species are those 

 given by Quenstedt under the name euryodos in his last great work ^; 

 but they afford no adequate idea of the developments of the fossil. 

 It may be noted that there are distinct differences in coiling and 

 ribbing between the specimens depicted in his fig. 5 and figs. 4 and 6 

 — a matter to which I shall have to refer later (series y). 



The generic position of zigzag has been a matter of some un- 

 certainty. I^eumayr omitted it from his ' Systematik der Ammo- 

 nitiden ' ; ^ and, though the species has sometimes been placed in 

 Stephanoceras, yet the peculiar ornaments which doubtless suggested 

 to d'Orbigny the name zigzag are different from the ornaments of 

 Ste-pliajioceras ; and, therefore, some authors have placed the species 

 in Perisphinctes. 



There are only two sources from which it would be possible to 

 derive zigzag^ namely, Stephanoceras and Zurcheria. 



I have broken up some eight or nine specimens without very 

 much success, on account of their crystalline condition ; but the inner 

 whorls show a rather globose form, which soon acquires small knobs. 



1 ' Pal. fran9., Terr, jurass.' pi. 129, figs. 9-11. 



2 ' Der Jura,' pi. 63, figs. 20, 21. 



3 Quart. Journ. Geol. Soe. vol. xxsvii. (1881) p. 596. 

 * ' Ammoniten des Scbwabischen Jura,' pi. 74. 



^ Zeitschr. d. Deutsch. geol. Gesellsch. toI. xxvii. (1875) p. 854. 



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