562 
of color and occasional approximation of two last septa have been 
observed in any shells that have come under my observation, and, 
therefore, I have thought this change probably belonged to the 
later ephebic and not to the true gerontic stage. 
These facts show clearly that in this genus the least involute of 
existing species, Wautelus umbilicatus, is the most primitive and 
has characteristics repeated more or less in the young of the more 
involute Vautilus pompilius. This observation is of great import- 
ance in this paper, since it confirms the opinion that genetic groups 
of Nautiloids and Ammonoids are series of parallel morphic modifi- 
cations, in the evolution of which the shells progressed from less 
closely coiled and less involute to more closely coiled and more 
involute shells. 
Dr. Beecher has called my attention in his preparation of /Vawt- 
lus. macromphalus to avery important fact in connection with the 
bioplastology of the Nautiloids, viz., that there are indications in 
the ontogeny of this species of degenerative changes which have also 
taken a parallel course to those observed in other genera and families ; * 
in other words, that it is more closely coiled and more involute in 
the nepionic stage than later in life. 
The nepionic stage of this species differs in form from that of 
Nautilus pompilius, but the most marked distinction lies in the 
abrupt bending of the shell in building the gyroceran curve. This 
consists partly in the formation of thick extensions of the shell 
along the lines of involution. These are similar to the testaceous 
umbilical extensions occurring in the same situation in JVawézlus 
umbtlicatus, but begin later in the ontogeny of that species, and 
also similar to those occurring in the young of WV. pompilius earlier 
than in wmdelicatus, but later than in the ontogeny of macrompha- 
dus. All of these facts and also the form of the young of macrom- 
phalus can only be accounted for by assuming that it is probably a 
descendant of Wautilus pompilius, which exhibits an accelerated 
development of earlier nepionic substages and then in the ephebic 
stage becomes less involute. 
I shall try to put these propositions in future papers into convinc- 
ing form with illustrations, but it is easy to verify them with any 
good specimen of JV. macromphalus since the internal whorls are 
visible in every case. 
These observations confirm in the most unexpected manner the 
generalization deduced from fossil shells, that in progressive series, 
