THE FOUR-GILLED GROUP 3309 



ranchiata, on the ground that the protoconch resembles that of Spirula rather than 

 that of the Nautiloidea. Others again regard the Amtnonoidea as a third and 

 distinct order of Cephalopoda. Their distribution extends from the Silurian to (pos- 

 sibly) the early Tertiary. No trace has ever been found of an ink sac, mandible, or 

 hooks on the arms; and the shell was undoubtedly external." 



