part 1] ox THE :sroKPHOLotTi: of the ammonite septum. 51 



embrvonic characters which have disappeared as the study of the 

 suture-hnes alone seems to suggest, hut to a bringing to the surface 

 once more of those embryonic features which, even in acmic 

 radicals, have persisted into adult life hidden from view by a narrow 

 fringe of complicated frilling. During retrogression this fringe is 

 gradually lost, and thus a })rocess analogous to the making of 

 septal sections takes place. Strictly, the sutures of senile stocks 

 should be compared, not with those seen in the ontogeny of 

 ancestral tyj^es. but with the septal sections of the ancestral adult 

 septum. It is not at all unlikely that, while ontogeny smnmarizes 

 ])rogressive evolution, septal sections forecast in similar broad out- 

 lines retrogressive evolution. Thus, for example, a careful study 

 of a large number of Baculites. and a comparison with the septal 

 sections of ammonites from which they have pi-obably descended, 

 would make it possible to connect up with certainty many of these 

 forms. It is to be regretted that inability to secure sufficient 

 material has made it impossible for us to establish this principle 

 satisfactorily. « 



XV. Asymmetry in Sutuee-Lines. 



A marked tendency to asymmetry in suture-lines has already 

 been noted in Dacfylioceras commune. It is now proposed to 

 deal with this aspect of the morphology of the septum more fully. 

 Such asymmetry usually arises in one of two ways, namely : — 



(1) By the different development of elements of opposite sides. 



(2) By association with lateral displacement of the siphuncle. 



The former is of common occurrence. It has already been 

 described above for Dactylioceras (p. 40). It has been observed 

 by S. S. Buckman in Sonninia ^ and by Solger in Neopty- 

 ckites? In each of these cases the first lateral lobes of opposite 

 sides were different in plan. Other cases have been seen in which 

 this character has been associated with a tendency to helicoid 

 coiling, as in the ' Turrilltes ' of d'Orbigny.^ As O. Fraas has 

 shown,* it is oriW when this character has become constant, as 

 in Turrilites, that it is of generic importance. 



Asymmetr\" associated with the displacement of the siphuncle 

 may be brought about in various wa^^s. The most instructive 

 example that has come under our notice is that of a specimen 

 of Ferispliinctes friplicatus, found in the Kimeridge Clay of 

 Market Rasen by Mr. P. T. Ingham. This siDecimen shows three 

 septa (PI. IV, fig. 2) ; the suture of the earliest is normal, that of 

 the second is slightly irregular, that of the third is markedly 

 distorted, for the siphuncle has during this short interval become 



' ' Inf. Ool. Ammonites ' Monogr. Palseont. Soc. 1894, p. 381. 

 ^ ' Die Fossilien der Mungokreide in Kamerun & ihre Geologische Be- 

 deutung' Beitr. Geol. von Kamerun (1904) p. 107. 



^ ' Paleontologie Fran(,'aise : Terrains Cretaces ' vol. i (1840) p. 571. 



^ ' Abnormitaten bei Ammoniten ' Jahreshefte Wiii'ttemb. xix (1863) p. 112. 



