34 PROF. H. H. SAVI^'XERTOX AXD ME. A. E. TErEMA>^ [vol. Ixxiii, 



fig. Q), the ventrally situated folds of the external series come into 

 line with their fellows of the opposite sides, and consequently 

 the elements of the venter exceed those of the dorsum b}^ the 

 number of folds which thus cross the central area.^ 



V. Teemixology of Loees axd Saddles. 



Some of the existing terminologies for lobes and saddles of the 

 suture-line caimot be adopted for the present purpose, because they 

 are to a great extent arbitrary. Thus the term superior, as 

 applied to the first two lateral saddles., although it tits in with 

 the accepted conventional method — used even in our own figures — 

 of representing the septum, is quite wrong when the septum is cor- 

 rectly placed with its venter downwards, in the attitude which 

 it is believed to have assumed when the animal was alive. 



The terminology suggested by Mr. S. S. Buckman - is most 

 nearly in accordance with the facts of development, and Avill be 

 adopted here. He gives the name of external saddles to the 

 pair on each side of the A^entral lobe; and uses the term lateral 

 for the others. The difference thus implied is a real one : the 

 external saddles originate fi'om the large median saddle (' aussen- 

 sattel' of Branco) of the first septum by the appearance upon its 

 crest of a median lobe (Spath, op. cif. 1914, p. Ji41 ; A. E. True- 

 man^). The lateral saddles, on the contrary, appear as simple folds 

 on the umbilical angle, and travel thence towards the venter 

 (PI. Ill ; and Spath, o;;. cif. pi. xlix, figs. 2a k 2 b). 



The development of the folds on the dorsum apjDarently follows 

 the same lines as those just laid down for the venter. Thus, the 

 first septum shows occasionallv a median saddle (fig. 12 a, p. 46 ; 

 Spath, op. cit. pi. xlviii, fig. 2 c). In the second septum this has 

 given place to a median dorsal lobe flanked by the internal saddles. 

 The remaining elements appear at the umbilical angle, and travel 

 towards the median dorsal line (see figs. 9 c, d, k e, p. 42 ; 12 c k 

 e, p. 46 ; and l.j c, d,f, &. g, p. 48 ). 



Although the distinction into laterals and auxiliaries may be 

 convenient for descri])tive jku'poses, it does not correspond to any 

 uiorphological difference between the folds — inasmuch as the 

 auxiliaries also appear for the first time as simple folds on the 

 umbilical wing of the septum, and travel towards the venter or 

 towards the dorsum, as the case may be. Moreover, the auxiliaries 

 of one ammonite may correspond to the laterals of another. 

 Thus, develo])ment shows that the large second lateral of Trago- 

 jihj/lloceras Iosco ntlji is the strict homologue of the small first 

 auxiliary of Dacfylioceras. 



^ The septal aponeurosis of amuionites does not seem to have been so well 

 developed as in Nantilut;. In them the fibres were apj^arently isolated and 

 attached to the shell along a wavy line at points indicated on the suture -line 

 by denticles. Pressure upon a membrane thus attached by its margin must 

 produce folds at i-ight angles to the peripherj'. 



2 ' Yorkshire Type Ammonites " vol. i (1909-12) p. ix. 



^ ' The Lineage of Traf/oj^h ylloceras loscombi ' The Naturalist, 1916, p. 222. 



