L. F. Spath — Notes on Ammonites. 121 



of which the Ammonite septum was composed was probably of the 

 same character and the same strength per unit cross-section as it is in 

 the living Nautilus, and similar!}'' deposited on a conchiolin membrane, 

 starting in the region away from the siphuncle. Pfaff l found that 

 in many well-preserved Gault Ammonites the pearly substance was 

 quite similar and showed an identical structure of more or less 

 parallel thin lamellae, as in the septum of Nautilus pompilius. This 

 author also shows in detail how in the case of Nautilus where the 

 septa are concave forwards, as well as in Ammonites, external excess 

 pressure would be transferred to the shell wall ; and he comes to the 

 conclusion that the arched septum of Ammonites need be only 

 one-sixth of the thickness of the septum of Nautilus. ButPfaff 2 

 further assumes that one reason why the suture-line of N pompilius 

 is only curved as compared with the highly ramified suture-line of 

 Ammonites is the thickening of the end-septum of the former. This 

 is improbable, however, for as all the previous septa were capable of 

 withstanding the external excess pressure, there was no need to 

 thicken the last one, and it seems to the writer that this thickening 

 is on a par with the occasional approximation of the last few septa 

 when the animal had reached its full growth. 



In Nautilus the siphuncle occupies exactly the centre of the 

 supporting septal surface, though, as Dr. Foord has pointed out, 3 

 a change of position of the siphuncle during ontogeny is of frequent 

 occurrence in fossil Nautili. The central siphuncular structure of 

 these acted not only as a strengthening feature, as suggested already 

 in 1854 by Pictet, 4 who thought it probable that in Ammonoids 

 the complicated septa "were necessitated by the excentric position of 

 the siphuncle'', but the siphon also afforded an additional means of 

 attachment. 5 In primitive Goniatites the earliest few septa are 

 concave, and it has already been stated that the second septum of 

 e.g. Dactylioceras also tends to be concave and is similarly associated 

 with a simple suture-line and a sub-central siphuncle. When it is 

 further found that e.g. in the Carboniferous Sulelymenia evoluta 

 (Phillips), the Triassic Clydonautilus goniatites (Hauer) or the Upper 

 Jurassic Pseudonautilus Geinitzi (Pictet) a nearly external siphuncle 

 is associated with an angular suture-line, it seems, indeed, probable 

 that the shifting of the siphuncle to the external side first started 

 the differentiation and convexity of the septal surface. 



1902; also B. Euedemann, "Structure of some Primitive Cephalopods " : 

 Eeport of New York State Pal. 1903, p. 334, for Piloceras.) They were 

 probably bentbonic, and it was only after the shell had become coiled upon 

 itself and bilaterally symmetrical, that the Cephalopod animal could adopt 

 a freely swimming mode of life. 



1 " Uber Form und Bau"d. Ammonitensepten und ihre Bezieh. z. Sutur- 

 Linie " : 4. Jahresber d. Niedersachs. Geolog. Ver., 1911, p. 212. 



2 Op. cit., p. 212. 



3 Cat. of Foss. Ceph. in the Brit. Mus. (Nat. Hist.), pt. ii ; Nautiloidea, 

 1891, p. 322. 



4 TraiU de Paleont., vol. ii, pp. 666-7. 



5 Older authors (e.g. Vrolik & Van Breda, Ann. Mag. Nat. Hist., vol. xii, 

 p. 173, 1843) even held that the animal was attached to the shell only by 

 the siphon. 



