4i8 PRiMAkY FACTORS OF ORGANIC EVOLUTION. 



Jura before the habit of close coiling could have acted 

 upon the whorls so as to produce this modification is, 

 therefore very general and convincing. 



"The leading characteristic of parallelism in all 

 genetic series of nautiloids is, as may be inferred from 

 the facts cited, a tendency toward closer coiling and 

 greater involution in the more specialized forms of 

 each separate series and a correlative increase in the 

 profundity of the impressed zone. When the impressed 

 zone becomes inheritable in some closely coiled and 

 involute specialized shells of the Carboniferous and 

 in similar shells in all of the genetic series of the 

 Jura this result is also directly connected with the ob- 

 served fact of the quicker development of the coiling 

 up tendency in the young of these Jurassic shells. This 

 is shown by the small diameter of the umbilical per- 

 foration in the centers of the shells of the Carbonif- 

 erous. It is also connected with the fact that the prim- 

 itive uncoiled forms, orthoceran, cyrtoceran, and gy- 

 roceran shells begin to die out in the Carboniferous 

 and cease with the Trias as mentioned above. 



"This demonstration of the characters that accom- 

 pany progress in close coiling, enables me to fill a gap 

 which occurs in the evidence during the Cretaceous. 

 In this period the existence of the impressed zone dur- 

 ing the cyrtoceran stage of individuals has not been 

 clearly established by observation except in two spe- 

 cies, a form allied to Cymatoceras pseudoelegans D'Or- 

 bigny, from Faxoe, and Cymatoceras elegans from Rouen. 

 In other shells, although a considerable number have 

 been broken down, the state of preservation has been 

 invariably imperfect. The coiling, however, in the 

 young of all the shells examined is notably more ac- 

 celerated than in the similar shells of the Jura, and the 



