120 L. F. Spath — Notes on Ammonites. 



strength to the shell to resist the pressure", the " further use 

 suggested by von Buch of affording points of attachment to the 

 mantle", showed to him "the union of two beneficial results from 

 one and the same mechanical expedient". Zittel 1 and Uhlig 2 

 held that the strongly ramified borders of the septa serve to increase 

 the solidity of the shells, and the latter author showed (for Lytoeeras) 

 that "as the whorls only just touch one another they could offer little 

 mutual support, and therefore every increase of resistance, if only 

 slight, must have been of great value. Thus, physiologically, the 

 sutural lobes would have served the same purpose that was attained 

 in another genus with very e volute whorls, Arietites, by the 

 external keel with its two accompanying grooves ". It may be 

 pointed out here that the septal surface, with excentric siphuncle., 

 in both Ammonoids and Nautiloids, in spite of its anterior and 

 posterior folds, is nearly enough at right angles to the whorl- 

 surface to act as a strengthening feature. In the adult septum of 

 Bactylioceras, fifty-two per cent of the area of the septum lies 

 between contours only '75 mm. apart, according to Swinnerton and 

 Trueman (p. 32) ; and their graph shows that the area of the 

 posterior folds is not much greater than that occupied by the 

 anterior folds, i.e. that the convexity is comparatively slight. 



In Nautiloids, provided with a simple suture-line, the shell in 

 general is considerably thicker than in Ammonoids, where, 

 ordinarily, it is " as thin as paper ", 3 Diener 4 writes : " The shells 

 of Phylloceratidas, Perisphinctidse, and Arcestidse are always delicate, 

 smooth shells of thickset build, more delicate than Ijjie Nautilus 

 shell ; Lytocerates are often as thin as paper and clear as glass, with 

 feeble ornament." Boehm 5 found that the shells of Nautilus, 

 washed up on the shores of the Sula Islands, were thicker than most 

 Callovian Ammonites. On the other hand, the formation of septa by 

 the nacreous layer of the shell and the periodical repetition of these 

 during the progress of growth, concurrently with the formation of 

 new layers which extend aud expand the mouth of the shell, was 

 probably the same in normal or irregularly coiled Ammonoids, as in 

 the ancestral cyrtocone 6 or the present-day Nautilus. The material 



1 Op. cit., 1881-5, pp. 332 ff. 



2 "Die Cephalopoden Fauna d. Wernsdorf, Schichten " : Denkschr. d. 

 Math.-Naturwiss, CI. d. k. Akad. d. Wiss., vol. xlvi, p. 61, Vienna, 1883. 



3 Op. cit., p. 332. 



4 Op. cit., 1912, pp. 67-89. 



5 "Beitr. z. Geol. v. Niederland. Indien " : I, 4, Paleeont. Suppl. IV, 1912,. 

 p. 173. 



6 Whether the earliest representatives were active benthonic animals' or 

 attached and sedentary, is not known. But it is probable that from the 

 ancestral capulicone, cyrtocones and orthocones arose, with elongation of the 

 shell after the manner of tubular structures in Actinozoa, Polyzoa, Annelida, 

 and Gastropoda ( "Guide to the Fossil Invertebrata Animals in the Department 

 of Geol. and Pal. in the Brit. Mus. (Nat. Hist.)," 1907, p. 147), and the 

 formation of septa at the end of the cone only after a continued period of 

 elongation and pulling away of the visceral hump from the cone, probably to 

 give it buoyancy. But orthocones and cyrtocones cannot have been active 

 swimmers. (See the interesting paper by O. Jaekel, " Thesen iib. d. Organis. 

 u. Lebensweise ausgestorbener Cephalopoden": Z.d.g.G., vol. liv, p. 67,, 



