PROCEEDINGS OF THE GEOLOGICAL SOCIETY. I 65 



1. " Notes on Nautili and Ammonites." liy S. S. Buckman, Esq., 



F.G.S. 



[Abstract.] 



1. The Position of the last Septum. — Mr. Bather's theory of shell- 

 growth in Cephalopoda (Ann. & Mag. Nat. Hist. 1888, i. p. 300) seems 

 to de])ond upon the idea that the last se])tum in the young of 

 I^autihdi and Ammonites was alwaj's formed at a proportionately 

 increased distance from the penultimate. A number of specimens 

 in the Author's collection present features of scptation contrary to 

 this idea. Of JS\(ufilas the Author exhibited six specimens, ranging 

 from young examples in which the first whorl was barely comjdeted, 

 to larger, but still immature individuals, all showing an undul}' 

 approximate last septum; and of yimmonites, a series of young 

 specimens belonging to the genera Witchellia, Lioceras^ Ludivirjia, 

 and Grammoceras presenting the same feature. The Author stated 

 that he did not put forward any theory to account for these facts. 



2. Shell-museJes of Nautili and Ammonites. — Some of the exhi- 

 bited specimens of Xautilus show the shell-muscles plainly ; and it 

 was pointed out that the curvature of the inner edge of the muscles 

 and of their connecting-ligament corresponds with the curvature of 

 the septum. Two specimens of Ammonites exhibited show long 

 spatulate depressions more or less parallel to the periphery for about 

 half the length of the body-chamber. It was suggested that these 

 impressions indicated the position of the shell-muscles. 



Discussion. 



Mr. Bather said he had always, since Oct. 1887, admitted that 

 .the last air-chamber in young specimens was sometimes relatively 

 shallow. But he denied that, had the animal lived on, this chamber 

 would have become deeper. Numerous specimens of the only available 

 recent Nautilus proved that no septum ever moved after its depo- 

 sition had once begun, and that approximation of septa was normally 

 a character of old age alone. He had, however, insisted on the fact 

 that "•' in shells of Nautilus and Ammonites a single shallow chamber 

 occasionally intervene[d], far back in the shell, between two of 

 normal size," and this could be seen in some of Mr. Buckman's own 

 specimens. This he had regarded as " a purely pathological 

 episode " due to a temporary slackening in the growth of the 

 animal. Such weakness, though it might be recovered from, 

 rendered the animal less able to resist unAivourable conditions ; 

 when an animal died young its shell naturally showed a trace of the 

 ■weakness, and thus he would explain those of Mr. Buckman's 

 specimens in which a]iproximation of the last septum could be 

 detected. He was therefore not inclined to depart from his pre- 

 viously jmblished opinions. 



Dr. Woodward remarked that he was glad to be in accord with 

 Mr. Bather with reference to the constant proportion observable in 

 the distance between the septa in the shells of both Nautili and 



