80 



THK GEOLOGIST, 



this epoch, when the animal had no longer the energy or 

 the faculty of reproducing the numerous ornaments with 

 which the shell was provided externally, it ceased also to 

 increase in a regular volume and degenerated in every res- 

 pect; thus from what has been already stated, there are 

 certainly three distinct periods in the progress of the spiral 

 convolutions of Ammonites; at the embryo, stationary or 

 adult and degenerate periods, we may readily conceive that 

 the adult state is the proper period for measuring an Am- 

 monite. 



It must be understood, that in these proportions, I am 

 only speaking of those whorls which are provided with 

 septa ; for the last two thirds of the convolutions, which is 

 always destined to receive the animal, and on that account, 

 unprovided with septa, rarely follows the regular curve, ex- 

 cept in certain particular species. It is almost always nar- 

 rower than the rest (as I have already mentioned in treating 

 of the mouth) and narrows gradually, from the last septum 

 to the mouth. 



It may perhaps be remarked that, in my descriptions, I do 

 not take the size of the last whorl, in comparison to the en- 

 tire diameter of the species, on the height of the central line 

 of the mouth, compared with the diameter, as if the Ammo- 

 nite were cut in two, but rather on the external portion of 

 the whorl, taken from the edge of the UmbiUcus, comparing 

 its size to the entire diameter. I find two great inconve- 

 niences in measuring the spiral curve mathematically on the 

 central line, or the comparative size of the first and second 

 whorl ; first, from being often obhged to break or cut the 

 specimen, to arrive at a correct measurement, and not being- 

 able to do so with specimens which were lent to me, I should 

 not on that account have been able to gather a number of 

 facts ; the second and more important one, consisted in 

 what I have observed that measurements are taken in this 



