i2 ILLINOIS STATE ACADEMY OF SCIENCE 



the Triassic and Jurassic periods the Ammonites with 

 very complex suture lines reached the climax of their 

 careers. Now at the apex of each of these Ammonite 

 shells the first sutures were simple, like those of the 

 adult Nautilus. These were followed in the first half 

 of the coil by sutures with simple lobes, as in Goniatites ; 

 and farther forward the lobes of the later sutures became 

 more complex, until the true Ammonite type of suture is 

 attained about the time the first whorl is completed. Thus, 

 each individual Nautilus shell repeats in its growth the 

 successive stages of curvature and coiling that the Nau- 

 tilus group passed through in its development from 

 straight-shelled ancestral forms; and in a similar way 

 each individual Ammonite shell repeats in its growth the 

 successive stages of complexity of suture line that the 

 Ammonite group passed through in its development from 

 the simple-sutured Nautiloid ancestors. 



In their growth the shells in many of the classes of 

 fossils repeat in their young stages adult characters of 

 their earlier ancestors so that the paleontologist does not 

 doubt the general validity of the evidence of the law of 

 recapitulation with regard to evolution. 



