﻿615 



dorsum of the gerontic volution is brought into contact with the 

 dorsum of the next younger volution and a contact furrow results, 

 which so far as I know occurs in all the species properly referred to 

 this genus, although very slight in some of them. 



The significance of the facts brought out by the study of degenera- 

 tive series has been fully discussed elsewhere, and need not be 

 noticed again. 



The facts and arguments brought forward seem to justify the fol- 

 lowing conclusions : 



i. The impressed zone is primitively a contact furrow, an ac- 

 quired characteristic of the dorsum of the whorls of nautilian shells 

 having large umbilical perforations, which appeared either in the ana- 

 neanic or metaneanic substages, and rarely later in their ontogeny. 

 There is abundant positive evidence that in these primitive forms 

 this furrow is a purely mechanical result of the nautilian mode of 

 growth, not appearing in the ontogeny before contact and either 

 partially or entirely disappearing on the free gerontic volution. 



2. The impressed zone does occur independently of contact on 

 the free dorsum of the paranepionic substage as a dorsal furrow in 

 some close-coiled, highly tachygenic, nautilian shells in the Quebec 

 group and in the Devonian. 



3. While there is no positive proof that the dorsal furrow origi- 

 nated through heredity in the paranepionic substages of these nauti- 

 loids of precarboniferous age, there is also no satisfactory evidence 

 that it originated in the young of such species as have this character 

 through purely mechanical agencies. 



4. There is positive evidence that the similar dorsal furrow which 

 also appears at the same age in the young shells of Coloceras glo- 

 batum and perhaps Coelogasteroceras canaliculatum among Carbon- 

 iferous nautiloids can be explained only when it is considered as a 

 transmitted, tachygenetic characteristic. 



5. This fourth conclusion is supported by the presence of a similar 

 dorsal furrow in the paranepionic substage of the young shells of all 

 of the nautiloids of the Jura, so far as observed. 



6. The fourth and fifth conclusions are rendered still more proba- 

 ble by the presence of the dorsal furrow at an earlier age, the meta- 

 nepionic substage, in all of the nautiloids so far as observed, from 

 the beginning of the Cretaceous, through the Tertiaries to and in- 

 cluding the living species of the genus Nautilus. Its presence on 

 this cyrtoceran volution in Cretacic shells can be explained only 



