iS93- 
107 [Hvatt. 
There is a notable exception to tliis rule when in highly taohy- 
genic shells the zone of impression is inherited and the dorsum 
becomes furrowed before the whorls touch. This is one of the 
most complete demonstrations of the probable inheritance of 
acquired characters that I know, and an excellent illustration of 
the law of tachygenesis. It occurs in some groups of nautilian 
shells of the Carboniferous and also in the Jura and Cretaceous in 
some forms and in Nautilus pompilius to a somewhat less degree. 
In tracing out the distinct phyla to which these different forms 
belong it can be shown that the impressed zone is invariably 
consequent upon close coiling, never appearing in ancestral forms 
in the nepionic period but always later, usually in the ananeaiiic 
substage of these more generalized and less closely coiled shells. 
Then rising in the series to the more s])ecialized nautilian and 
deeply involved shells this purely acquired chai-acter becomes 
through the action of tachygenesis forced back into the nepionic 
stage liefore the wliorls touch. It is therefore in these forms 
entirely independent of the mechanical cause, the ])ressure of one 
whorl upon another, which first originated it. One needs only to 
add that this configuration of the dorsum is never found in adults 
of any ancient and normally uncoiled shells so far as I know, nor 
so far as they have been figured.^ 
The ananeanic substage among Carboniferous cephalopods is 
not only marked by the beginning of the impressed zone but also 
as a rule l)y the introduction of correlative changes in the form of 
the whorl. Thus the hexagonal whorl Avith an outline similar 
to that of an inverted trapezoid in section and consequently an 
obvious repetition of the ephebic whorlof Temnocheilus, and with 
sutures also like those of the adults of that genus, appears at this 
stage in Carboniferous ce]»halopods of several different genera, 
showing their immediate descent from Devonian Temnocheili. 
The first appearance of the annular lobe in the dorsal sutures is 
correlated with closer coiling and is apt to make its first appear- 
ance in primitive )iantilian shells at this stage in the impressed 
zone. This annular lobe occurs also before the whorls touch in 
Nautilus pompilius, but whether it is strictly correlative with the 
impressed zone in obeying the law of tachygenesis in Carbonif- 
erous forms has not yet been ascertained. 
' "Phyliiiienv of an acquireil cliaractrristii-" Aincr. nat., 1893. 
