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In Nostoceras similar phenomena are observable but, in this tur- 

 rilites-like, closer coiled spiral, the young shells are quite different 

 and it is not certain that they are irregular and similar to Hamites. 

 The species of this genus and of Emperoceras and Didymoceras 

 show that the spiral coiled stage is an ephebic stage, not a true 

 gerontic stage of the ontogeny, because passing beyond this the 

 gerontic stage appears taking on the usual retro versal form. The 

 ephebic whorl departs from the spiral in this stage, again becoming 

 excentric, and then builds back towards itself and towards the 

 spiral, forming the peculiar crook found more or less in the parage- 

 rontic substage of the so-called Hamites, Ancyloceras, Scaphites. 



Thus one gets in these two genera a demonstration that the tur- 

 rilites and helicoceran modes of building the shell are acquired 

 characteristics of the ephebic stage of the ontogeny interpolated 

 between gerontic and neanic stages which have the usual charac- 

 ters of these stages in the ontogeny of degenerate forms. 



These forms are also interesting in connection with the history 

 of the impressed zone, because if they have close-coiled young, like 

 those of the crioceran and baculites-like shell already studied, 

 which is highly probable, they must have had a contact furrow in 

 the nepionic stage and then lost it in the neanic stage. The gene- 

 sis of another contact furrow in the still later stages of Nostoceras 

 and similar turrilites-like spirals, is therefore secondary and phylo- 

 gerontic, and is not strictly speaking a progressive characteristic. 

 This furrow is also situated on the lateral aspect and not on the 

 dorsum as in symmetrical shells. 



The phylogerontic renewal of the impressed zone is also in Pty- 

 choceras, a generic character, as pointed out to me by Mr. T. W. 

 Stanton, to whose courtesy and the kind permission of Mr. C. D. 

 Walcott, Director of the Geological Survey, and Mr. Goode, Direc- 

 tor of the National Museum, I owe the fine materials described 

 above. 



The return of close coiling in gerontic stages of this species is a 

 remarkable phenomenon. There is a gerontic umbilical perfora- 

 tion formed by the sudden bending of the gerontic living chamber 

 which is elongated and not usually very small, but the gerontic 

 bend is often very abrupt. The inner side at the bend is occupied 

 by a gerontic dorsal furrow which reminds the observer of the dor- 

 sal furrow in the paranepionic substage of the coiled young of 

 Nautiloids. As in the young of Trocholites and Tarphyceras the 



