606 
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 ghe 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 Vautzlus um- 
bilicatus, the most involute Mautilus pompilius and the degenerate 
shell of Mautilus macromphaius. 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 Wautilus umbilicatus, but so far as I could see the dorsal 
furrow appeared in this shell quite as early as in Wautelus 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 mautilian 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 
