68 L. F. Spath — Notes on Ammonites. 



from the Upper Cretaceous Rocks of Zululand. Each showed 

 approximation of the last three septa . . . One specimen showed 

 also irregularities of depth in the other chambers of the camerated 

 part of the shell". Foord 1 figures two Inferior Oolite Nautili 

 {N. pseudolineatus and N. polygonalis), one of which has the last 

 septum closer than the usual interval, the other has it more distant. 

 A fair amount of variability as regards the spacing of the septa 

 (especially the last) is also found in Palaeozoic genera, e.g. the 

 Middle Devonian forms referred to "Gomphoceras" by Cleland. 2 In 

 Wisconsin this genus is found not only in great abundance as 

 regards the number of individuals, but is also represented by 

 a variety of more or less closely related species, Avhich rather 

 suggests favourable conditions. In Bohemia also, where the 

 Silurian period produced an exceedingly rich Cephalapod fauna, 

 this variability prevails, as is shown in Barrande's classical work. 3 



As regards Ammonites, the irregularity is even more striking. 

 A. E. Trueman 4 has recently figured some Polymorphites that show 

 approximation of the last septa. This seems to be an unstable genus 

 of generally small and very variable forms of limited horizontal 

 distribution. But in Hildoceras bifrons, which species-group, with 

 horizontal variants and vertical mutations, is found in North - 

 Western Europe, in the Alpine-Mediterranean-Pontian province, and 

 as far as Japan, the phenomenon is observed, as well as in its 

 probably benthonic and often one-sided derivative Frechiella. Among 

 fifty-six specimens of Amioceras niger (Blake), i.e. of another 

 universal genus, that the writer examined, six specimens had their 

 suture-lines fairly distant, and in five more the distance was some- 

 what less. Twenty-seven specimens had the septa a medium 

 distance apart; in five they were fairly close, and in thirteen very 

 close together. Besides, there were irregularities in almost every 

 specimen, many of them showing closer septation after a fairly 

 distant beginning. 



On thirty-one specimens the last sutures were well displayed, and 

 here even greater irregularity was noticed. In one specimen there 

 was no approximation at all, and in two more it was only just 

 noticeable. Nine specimens had the last two suture-lines fairly 

 close, in three more they were very close. In eight specimens there 

 was approximation of the last three septa; in one the last four 

 suture-lines were fairly close, and in another they were very close 

 together. One specimen had the last five septa closer than the 

 previous ones, but they were equidistant from one another, whereas 

 in another specimen the last five septa were gradually approxi- 

 mating. Two more specimens had the last six suture-lines gradually 

 getting closer, and finally, in another two there was first 



1 Op. cit., vol. ii, p. 214.' 



2 "The Fossils and Stratigraphy of the Middle Devonic of Wisconsin": 

 Wise. Geol. and Nat. Hist. Surv., Bulletin No. xxi, 6, 1911. 



3 Systime Silurien de la BoMme, vol. iii (Cephal.), 1867-77. 



* "Observations on the genus Polymorjrfiites ' ' : Geol. Mag., N.S., 

 Dec. VI, Vol. IV, pp. 443-4, figs. 1, 11, October, 1917. The writer cannot 

 accept the derivation of this genus from the much earlier arietid Agassiceras 

 development, Cymbites, and will refer later to the resemblance in the sutural 

 development. ' 



