HEREDITY. 421 



" 3. While there is no positive proof that the dor- 

 sal furrow originated through heredity in the parane- 

 pionic substages of these nautiloids of pre-Carbonife- 

 rous 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 dor- 

 sal furrow which also appears at the same age in the 

 young shells of Coloceras globatum and perhaps Coelo- 

 gasteroceras canaliculatum among Carboniferous nauti- 

 loids can be explained only when it is considered as a 

 transmitted, tachygenetic (accelerated) characteristic. 



"5. This fourth conclusion is supported by the 

 presence of a similar dorsal furrow in the paranepionic 

 (adolescent) substage of the young shells of all the 

 nautiloids of the Jura, so far observed. 



"6. The fourth and fifth conclusions are rendered 

 still more probable by the presence of the dorsal fur- 

 row at an earlier age, the metanepionic substage, in 

 all of the nautiloids so far observed, from the begin- 

 ning of the Cretaceous, through the Tertiaries, to and 

 including the living species of the genus Nautilus. Its 

 presence on this cyrtoceran volution in Cretacic shells 

 can be explained only when it is considered as a trans- 

 mitted, tachygenetic (accelerated) characteristic de- 

 rived from ancestral nautilian shells of the Jura, which 

 have the same characteristic at a later age, i. e. , in the 

 paranepionic substage. 



"7. The first conclusion is also sustained by the 

 parallel phylogeny of the impressed zone in the ances- 

 tral forms of the Ammonoidea, the Nautilinidae, and 

 especially in the Mimoceras, the radical genus of this 

 family. 



"8. The fourth, fifth, and sixth conclusions are 



