28 L. F. Spath — Notes on Ammonites. 



Blake's suggestion is adopted, that this forward convexity of the 

 later septa (so conspicuous only because the average section of an 

 Ammonite happens to pass through the ventral and dorsal lobes) is 

 evidence of pressure from behind the animal; and it is assumed that 

 "in Ammonites the vigour of secretion may have been so great 

 that the gas exerted sufficient pressure upon the soft mantle to make 

 it bulge forward while the septum was being deposited" (p. 33). 



The influence of this pi'essure is referred to in connexion with the 

 modification of the adult suture-line in Dactyloidse, where among 

 other "ageing" characters of the later suture-lines the authors 

 mention the " more intricate wrinkling of the minor details". They 

 state (p. 39), "This complexity is strongly suggestive of the 

 wrinkling of a collapsing or flaccid bladder, as opposed to the 

 simpler and more turgid outlines of the folioles in earlier septa, and 

 suggests a diminution in the vigour of gas-secretion in the declining 

 period of life." The association of complexity with decline may 

 seem contradictory ; for the authors, speaking phylogenetically, say 

 (p. 51) "during retrogression this fringe [of complicated frilling] is 

 gradually lost". This complexity is not so apparent, however, as 

 the other " ageing " characters mentioned, namely the " decrease in 

 the antero-posterior range of the lobes and saddles" or "crumbling 

 down of the apices of the saddles to approximately the same 

 plane", and the "swinging forward of the umbilical portion of the 

 suture-line" (p. 39). Following S. S. Buckman, the authors consider 

 the Dactyliocerates to be " evidently a decadent offshoot of 

 Cosloeeras" ', though to the writer neither the decadence of this most 

 flourishing family nor the derivation from the Carixian pettos-growp, 

 to which the genus Coelooeras must be restricted, is evident. They 

 see in them "the phenomena which characterize the first stages in 

 the simplification of the suture-line, a simplification that is carried 

 to such extremes in Cretaceous Ammonites" (p. 40). 



CoilEELATION OF SlTTURE-LINE AND WhOEX-SHAPE. 



Attention must be drawn in this connexion to the close relation- 

 ship that exists between the suture-line and the shape of the whorl. 

 In the Dactyl oidse the tendency is towards loosely coiled, more or 

 less cylindrical whorls, and in the evolution from a cadicone 

 ancestor, through depressed whorls, to the slightly involute shell of 

 the Dactylioceras figured by Swinnerton & Trueman, the suture-line 

 would adapt itself to the altered whorl-shape. Zittel 1 stated: 

 "When the whorls are circular one observes ordinarily only a few 

 lobes, and in that case they are of nearly equal dimensions 

 (Lytoceras); upon a wide ventral area the external lobe and external 

 saddle acquire considerable dimensions; the flatter the sides are and 

 the thinner the ventral part, the larger the size of the lateral lobes 

 and lateral saddles, and the more numerous the auxiliary lobes." 

 Pfaff 2 mentions that "compressed forms would show the greatest 

 differentiation of their suture-lines in the lateral region and in the 



1 Handbuch d. Pal., vol. i, 2, pp. 332, etc., 1881-5. 



2 "Form u. Ban d. Amnion. -Sept., etc.": Jahresb. Niedersachs. Geol. 

 Ver., vol. iv, pp. 221-2, 1911. 



