122 L. F. Spath — Notes on Ammonites. 



The writer would like to draw attention in this connexion to the 

 interesting series of Nautili from the London Clay, which shows the 

 close interconnexion of the various mechanical features of the shell. 

 Nautilus Parkinsoni, Edwards, which has Aturia lobes (but the 

 siphuncle farther away from the dorsum than e.g. thegaleate-whorled 

 N Soiverlyi, "Wetherell, with only slightly undulating septal edges) 

 lacks the wide trumpet-mouthed funnels of typical Aturice where the 

 siphuncle is dorsal. It might be suggested that apart from its 

 connexion with the attachment of the animal to its shell, the 

 differentiation of the septal surface in the neighbourhood of the 

 siphuncle would afford protection for the latter. This would apply 

 especially to the typical Ammonites in which elaboration of the 

 suture-line begins at the siphonal lobe and progresses dorsally as is 

 shown, e.g. in fig. 9 (p. 42) of Swiunerton & Trueman's paper, and 

 in which external features such as a keel or a groove are similarly 

 interpreted as contrivances for the protection of ihe siphuncle. 3t 

 is probable, however, that this is partly also a case of retention of 

 an original feature and gradual elaboration of the first ventral lobe 

 or siphonal collar of the Goniatite radical, in such a manner that in 

 a later Ammonite, e.g. the external saddle of stage 1 = the first 

 lateral saddle of stage 2 = the second lateral saddle of stage 3 = the 

 first auxiliary saddle of the (latest) stage 4. 1 



Pfaff 2 states that " as during growth the septal surface increases at 

 the relatively quickest rate on the external side, differentiation must 

 begin here". But in that case Clynienidse, with an internal 

 siphuncle, should not show greater elaboration of the suture-line on 

 the dorsum. Here, also, protection for the siphuncle would have to 

 be assumed, and the abnormal position of both siphuncle and deeper 

 lobes suggests derivation of this stock, not from Gyroceras as Freeh 

 thinks, but from a Goniatite ancestor with a siphuncle, the position 

 of which may have been unstable (as in the early whorls of most 

 latisellate Ammonoids) but which had already become associated with 

 the region of greatest differentiation of the septal surface. 



It is necessary to distinguish between the progressive elaboration 

 of the suture-line in the whole order Ammonoidea and the 

 differentiation in certain stocks necessitated by e.g. whorl-shape. 

 The Dimorphoceras-Thalassocernsliueage, e.g., seems to be the first one 

 in which the minor details of the ventral lobe are elaborated, and 

 from Pronorites onwards, and especially during the Permian and 

 Lower Trias, first the lobes and then the saddles of all the stocks 

 show progressive frilling. On the other hand, in more specialized 

 lineages, such as the Devonian Beloceras, already differentiation is 

 most pronounced in the lateral region, though it begins with the 

 primitive ventral lobe. This applies to practically all compressed 

 Ammonoids, as has already been stated, and demonstrates the 

 impossibility of a morphological classification that groups together 

 the above Devonian Beloceras and the Upper Triassic Pinacoceras, 

 simply because these heterochronous homoeomorphs possess a 

 compressed shell. 



1 SeeL.F. Spath, Q.J.G.S., vol. lxx, fig. on p. 341, 1914, stages e,/, h, and I. 



2 Op. cit., p. 222. 



