608 
that gerontic degeneration is slight and does not affect the amount 
of involution nor the size of the whorl. This may be due to the 
rarity of shells that have reached an advanced age or to the brittle- 
ness of the senile volution, but against this there is sufficient evi- 
dence. 
Thus, in many Mesozoic fossils and in recent Nautili, shells are 
often found with the last two or three septa approximating and this 
is plainly a mark of the failure of the powers of growth and shows 
in most examples of large size that the animal has probably reached 
the extreme limits of its existence. 
One fact is of great interest in this connection. Extreme cases 
of degenerative series are rare among Nautiloidea. The Lituitidz 
stand alone as the only complete series that can be compared with 
several that are found among Ammonoidea. The Discoceratide 
_ have also some turbinate genera that can be closely compared with 
the helicoidal spirals of a number of Ammonitinze. All such forms 
and others that may be supposed from their characteristics to exhibit 
similar characteristics, disappear with the Paleozoic and all, so far 
as I know, before the Carboniferous period. ‘There are phyloge- 
rontic species like Coloceras globatum in the Carboniferous, but no 
uncoiled phylogerontic forms. 
In Mesozoic, Tertiary and Cenozoic times, the uniformity of the 
type is conspicuous, and while it is plainly degenerating from the 
Carboniferous to the present, this process is not accompanied by 
the evolution of uncoiled series. The degeneration takes place as 
stated above in ornamentation of the shell and in the number and 
variety of the series and forms evolved, but not in the coiling, 
which is really progressive, nor yet in the sutures, since Aturia is 
certainly one of the most if not the most highly accelerated and 
specialized of the whole order. 
These facts all bear directly upon the history of the impressed 
zone, since in all uncoiled whorls the primitive contact furrow tends 
to disappear and the outer dorsal porcellanous layer is restored to 
its full development on that side. 
In Paleozoic time as well as in later times no involute shell has 
yet been observed with a free gerontic volution, that is to say, when 
the area of involution reached beyond the limits of the venter and 
the area covered extended inwardly on to the sides of the next 
inner whorls, the gerontic stage also remained involute, or, if 
decreasing in its ventro-dorsal diameters, this decrease never seemed 
