30 L. F. Spath — Notes on Ammonites. 



the ancestral stock. In Baeulites again, where, theoretically, the 

 suture-elements should he equal, there is a fair amount of variation, 

 caused probably hy the differences in cross-section, ornament, and 

 thickness. 



It may be added here that certain Lower Aptian developments of 

 the Upper Barremian Leptoceras (frispinosum-gvony) show how with 

 the gradual straightening out of the shell the sutnre-elements 

 become more nearly equal. One such form is e.g. " Hamites" 

 nodosus, v. Koenen, 1 which, of course, has nothing to do with the 

 Albian genus Hamites, just as " Bochianites " undulatus, v. Koenen, 2 

 cannot be attached to Bochianites, a homceomorphous development of 

 an earlier perisphinctoid Hoplitid. The suture-line of these hoplitid 

 Crioceratids then assumes the aspect of that of the lytoceratid 

 Maeroseaphitidae, and the distinction between these two important 

 families becomes very difficult. 



CORRELATION OF ScTTJKE-LINE, ATTACHMENT TO ShELL AND MODE 



of Life. 



Another factor has to be considered here. It appears probable 

 that in Ammonites, as iu the recent Nautilus, the shell-muscle could 

 easily be detached from the shell when the animal moved forward 

 a certain space to form a fresh chamber, and that, as there would 

 have been considerable risk of the shell falling away from its 

 inhabitant, the folded posterior portion of the Ammonite animal 

 with its lobes and saddles afforded the means of holding on, a function 

 performed in Nautilus by the strong central siphuncle. 3 When 

 a stock like Baeulitts, in addition to its tendency to equalize the 

 suture-elements of its straight shell, also shows simplification of the 

 suture-line, it may be assumed that it represents an adaptation to 

 a benthonic mode of life which its form alone would indicate. For 

 Lytoceras itself, "often as thin as paper and clear as glass, with 

 feeble ornament, i.e. characters that clearly remind one of adaptive 

 forms of nectonic Gastropods of the high seas, Atlanta" 4 and the 

 delicate and smooth shells of Phylloceratidae and Arcestidse do not 

 generally show simplification of the suture-line. In the latter, and 

 also in most oxynote shells, so admirably adapted for an actively 

 swimming mode of existence, the need for secure attachment of the 

 animal to its shell was probably greater than in benthonic crawlers. 



In those one-sided Ammonites of the Lias, called r Turrilites by 

 d'Orbigny, which, unlike Buckman 5 I would consider to be such 



characters and especially geological occurrence would appear to connect (in 

 this case TragophyUoceras and Bhacopkyllites) on the comparatively in- 

 significant difference in the endings of the external saddle. 



1 " Ammonitiden des Norddeutschen Neocom." : Abh. Preuss. Geol. 

 Landesanstalt, N.F., Heft xxiv. p. 393. pi. xxxv, fig. 13. 



a Ibid., p. 398, pi. liii, figs. 11, 13, 14'. 



s See E. A. Smith, "Note on the Pearly Nautilus," Journ. Conch., Oct.,. 

 1887; and Foord, Cat. Foss. CepJi. Brit. Mus. (Nat. Hist.), pt. i, pp. xi, xii, 

 18S8. 



4 Diener, " Lebensweise u. Yerbreitunsi d. Ammoniten " : N. Jahrb. Miner., 

 etc., ii, pp. 67-89, 1912. 



5 " Vererbunpsgesetze mid ihre Anwendung auf den Menschen " : Darwinist. 

 Schriften. I, vol. xviii, p. 22 (214), 1893. 



