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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, Nautilus umbilicatus, is the most primitive and 

 has characteristics repeated more or less in the young of the more 

 involute Nautilus 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 Nauti- 

 lus macromphalus to a very 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 Nautilus 

 umbilicatus, but begin later in the ontogeny of that species, and 

 also similar to those occurring in the young of N pompilius earlier 

 than in umbilicatus, but later than in the ontogeny of ?nacrompha- 

 lus. 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 Nautilus 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 N. 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, 



