614 
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 retroversal 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-cailed 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 
ee 
