AMMONITES — ZOOLOGICAL, ETC. 



79 



noticed ; their convolutions, and the connection of its modi- 

 fications with the other characters ; and the Hmits of growth 

 proper to the species. 



The spiral convolutions of shells [enroulement spiral) has 

 already occupied the attention of many scientific men, 

 amongst whom I may mention Messrs. Moseley of Cam- 

 bridge and Naumann of Freiburg, who have observed that 

 the spiral of shells follows logarithmic curves. M. Elie de 

 Beaumont^ by the suggestions of Leopold von Buch, has 

 successively measured a Goniatite and three Ammonites 

 from the Jurassic formation, and he has also found that they 

 were curved in spirals of nearly the same nature. Never- 

 theless, he observed that the spiral of two of these Ammo- 

 nites was a re-entering one, towards the termination of the 

 convolution, a little within the logarithmic spiral. Without 

 having studied the convolutions of Ammonites mathemati- 

 cally, I have observed in a great number of species that in 

 adults they are invariably the same, but that they vary two 

 or even three times according to age; 1st. If we take an Am- 

 monite in the embryo period, for example, at the instant 

 when the spiral commences, we shall observe that it increases 

 more rapidly in the first whorls, and departs from the 

 convolution which it ultimately arrives at. 2nd. After this 

 period which is usually very short, the convolutions become 

 regular and continue so in most cases, during the remainder 

 of the animal's existence. 3rd. At other times, as M. Elie 

 de Beaumont has observed, the spiral, instead of increasing 

 regularly, during the last period of degeneration, at the time 

 when the Ammonites begin to lose their last remaining ex- 

 ternal ornaments, re-enters, frequently within the regular 

 curve that it had hitherto followed. We should say that at 



* Societe philomatique de Paris 1841, p. 45, seance du 17 avril, 

 1841. 



