78 THE GEOLOGIST. 



sess and which 1 shall describe in my Fauna of the Jurassic 

 formation. 



If we found these apertares in Ammonites of the same 

 size, in each species, one would imagine that they belonged 

 to the adult age, and that they were formed as in C(/prea^ 

 at the time of the shell arriving at its greatest growth ; but 

 this is not the case ; and many examples have convinced 

 me that these apertures are found at all ages on individuals 

 quite perfect. We must believe then that, to increase its 

 shell, the animal is obliged to destroy this first margin, 

 which it afterwards reconstructs, when it becomes neces- 

 sary. This appears more proljable, as the open portion of 

 the last Whorl appears to be constructed provisionally, as it 

 becomes modified after forming chambers, as in most Am- 

 monites. I shall be able to demonstrate this in the Jurassic 

 formations, by many examples, where the growth is always 

 regular in the chambered portion ; whilst in that part in 

 which there are no chambers, it recedes, the last Whorl be- 

 comes smaller and has not, near the margin, more than half 

 the diameter of the parts which are entirely perfect. I wish 

 to draw particular attention to this singular character, which 

 will perhaps explain the strange forms of the other Genera 

 of Ammonites of which I intend to treat. 



We see then that the different forms of margins of Am- 

 monites coincide always with those of the back of the spe- 

 cies, according as this part is keeled or canaliculated, tuber- 

 culated, smooth or ribbed. These diversities in the form of 

 the back are of great value in forming groups in the Genus 

 of the Ammonites, in confirming many groups already 

 formed by Von Buch, and will serve me as a base for the 

 creation of many others, when I shall study Ammonites 

 with regard to classification. 



Before proceeding to the examination of the internal cha- 

 racters of Ammonites, two distinct subjects remain to be 



