300 Mr. F. A. Bather on Shell-groioth 



of yoimg sJiellSj as though basing an Induction with regard 

 to all on the evidence of this one. The intussusceptionist 

 might therefore contend that the shell in question was abnor- 

 mal, and would support his contention by the fact that in shells 

 of Nautilus and Ammonites a single shallow chamber occa- 

 sionally intervenes, far back in the shell, between two of 

 normal size. 



Even from a single shell, however, v. Lendenfeld might 

 have adduced confirmatory evidence had he grasped the full 

 significance of this specimen. His figure of the young shell 

 (12, t. ix. fig. 2) shows the last-formed septum partially 

 destroyed ; the destruction was due to its extreme tenuity ; 

 the portion left forms a rim round the shell- wall and Is much 

 thinner than the corresponding part of the preceding septa. 

 This septum was in fact only half formed when the animal 

 was killed. In other young specimens I had observed rims 

 of similar nature, but much narrower and thinner, as though 

 the remnants of septa in the very earliest stage of formation. 

 Such sutural rims show, whenever they occur, that the new 

 septum was from the beginning formed at the normal distance 

 from the penultimate septum. Moreover, In every young 

 Nautilus-shell examined by me (two in the Oxford Museum, 

 one in the British Museum, and two in my own collection), 

 and in the rare young Ammonites * that I found sufficiently 

 preserved, the interval between the last two septa is greater 

 than the preceding intervals in a constant proportion. All 

 these facts seemed to me conclusive ; but v. Lendenfeld^s 

 single argument from a single specimen proves nothing. 



Still he does happen to have hit on a typical example ; 

 and his figure, though rather obscure, Illustrates more points 

 in my paper (10) than in his own. Taking with gratitude 

 a favourable view of these figures, I may point out that 

 they are both reduced by one third from the original speci- 

 mens and that these are exhibited in the shell-gallery of the 

 Natural- History Museum. 



III. Refutation of Riefstahl. 



Von Lendenfeld avowedly refrains from criticizing the 

 main part of RIefstahl's paper, nor does he question the 

 Intussusception-hypothesis as applied to the sepion. Mere 

 comparison with Nautilus would not have justified such action, 



* In the examination of fossils I received much help from Messrs. E, 

 T. Newton and H. A. Allen, of the Geological Survey, for which I tender 

 my hearty thanks. 



