ANIMAL PEDIGREES 217 



tion. The group consequently is a very suitable 

 one to study from our present standpoint ; and the 

 enquiry gains additional interest from the fact that 

 Ammonites are an entirely extinct group of 

 animals, no single species having survived the 

 cretaceous period, so that our only chance of learn- 

 ing anything about their embryology is to study the 

 fossil shells themselves. 



Wurtenberger, who has made a special study of 

 the Jurassic Ammonites, has shown that there is 

 the same correspondence between historic and 

 embryonic development that obtains among living 

 animals. In the middle Jurassic deposits, for 

 instance, the older Ammonites are flattened and 

 disc-like, with numerous ribs ; in later forms the 

 shell bears rows of tubercles near the outer side 

 of the spiral, and later still a second inner row of 

 tubercles as well, while the ribs gradually become 

 less conspicuous and ultimately disappear. In 

 forms from more recent deposits the outer row of 

 tubercles disappears, and then the inner row, the 

 shell becoming smooth, swollen, and almost 

 spherical. On taking one of these smooth spheri- 

 cal shells, such as Aspidoceras cyclotum, and 

 breaking away the outer turns of the spiral so as 

 to expose the more central and older turns, 

 Wiirtenberger found first an inner and then an 

 outer row of tubercles appearing, which nearer the 

 centre disappeared, and in the oldest part of the 

 shell were replaced by the ribs characteristic of 

 the earlier, and presumably ancestral forms. 

 Results such as these open up to us a new field of 



