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showed the highest degree of tachygenetic development in all the 

 structural characters of progressive evolution among Nautiloids. 

 That is to say, the size of the apical chamber, the immediate 

 assumption of a highly matured outline in the first suture which 

 has the aturian generic lobes and ventral saddles, the subdorsan 

 siphuncle, the minute umbilical perforation and the rapid increase 

 of all the diameters of the apex in the nepionic stage and the 

 almost complete involution of the apex and first whorl in neanic 

 stage, all indicated a high degree of acceleration. It is therefore 

 probable that in this family a correspondingly early inheritance of 

 the dorsal furrow will also occur, unless there is some interference 

 arising from the highly tachygenic development of the character- 

 istics cited above in the metanepionic substage that may have re- 

 placed it or rendered it very obscure. Sections ought to have been 

 made to establish this fact, but I could not obtain materials for this 

 purpose in the limited time at my disposal. 



The existence of the dorsal furrow has been observed in the 

 metanepionic substages of the three existing species of Nautilus 

 that are the most important, viz. : the least involute Nautilus um- 

 bilicatus, the most involute Nautilus pompilius and the degenerate 

 shell of Nautilus macromphalus. It might of course be expected 

 that some of the less involute shells of the Cretaceous, Tertiary or 

 Present, if any such be found, would resemble the Jurassic shells in 

 having a dorsal furrow in the paranepionic. I expected this might 

 occur in Nautilus umbilicatus, but so far as I could see the dorsal 

 furrow appeared in this shell quite as early as in Nautilus pompilius 

 or macromphalus. 



I here take the opportunity to refer to the structure of the shell 

 of the dorsal side among Nautiloidea. 



The shell of course in all forms with free whorls is as complete 

 on the dorsal as it is on the ventral side. It is also complete on 

 the dorsum in the nepionic stage of all nautilian species. An 

 additional layer called by various names, but known in the modern 

 Nautilus as the black or dark-colored layer, makes its appearance 

 after contact and lies between the exteriors of the shells of the 

 venter and dorsum in each whorl. 



I have never been able to detect the homologue of this layer 

 among fossils probably because it is necessary to look for it in sec- 

 tions under the microscope. 



As regards the behavior of the shell in the impressed zone after 



