1893.] Phylogeny of an Acquired Characteristic. 871 
when, as is the case in extreme age, these cease to increase in 
size. Assoon as this senile contraction begins to occur the 
sides shrink, becoming narrower, the amount of involution 
becomes less, and the impressed zone, shrinks in breadth as 
shown in the sections. When the whorl finally parts com- 
pany in consequence of continued contraction the already 
shrunken impressed zone, figs. 4d, 5d, rapidly disappears and 
the apertures of such shells are frequently as round and free 
from indentations on the inner as on the outer side, as is shown 
at the free end of the figures 4 and 5. 
In normally uncoiled forms, usually named Lituites, when 
the adult or young is coiled, and the succeeding stages, 
whether representing adults or old shells, are uncoiled, the 
phenomena are similar. The impressed zone is lost after the 
growth ceases to bring the whorls of the shell into contact. 
The young and the adults of many of these forms have now 
been observed in the earliest periods and it is, therefore, obvi- 
ous that during these early times the impressed zone must have 
been a modification of the whorl which took place in conse- 
quence of the mechanical effects produced by close coiling. 
This characteristic is slight when the coiling is slight and is 
developed in precise proportion to the increase of coiling or 
involution of the whorls and, on the other hand, when through 
degeneration due to age, or to other causes, the whorls cease 
growing in contact, the impressed zone gradually disappears. 
Thus it always appears preceded and accompanied by an 
observable tendency in the mode of growth toward closer coil- 
ing and that this tendency is quite capable of producing the 
impressed zone can hardly be denied with any show of reason, 
since the charateristic disappears in proportion as the pressure 
is relieved through the degeneration of the powers of growth 
force to continue the normal rate of progressive increase of 
bulk in old or young or prematurely degenerate shells and in 
-uncoiled whorls of all kinds and all ages. 
The shells of Devonian series of Nautiloids have also been 
extensively examined, especially in the more involute nautil- 
ian forms of the genus Nephriticeras, and so far not one has 
been found with the slightest indication of the presence of an 
