407 
the impressed zone. This is the name I have given to the area on 
the dorsum affected by the contact of the dorsum of the growing 
whorl with the venter of the already formed whorl of the next inner 
volution. This is either flat, gibbous, or indented in accordance 
with the form of the venter of the whorl it touches or envelopes, 
but it is usually indented more or less deeply. 
There is a notable exception to this rule when in highly tachy- 
genic shells the zone of impression is inherited and the dorsum 
becomes furrowed before the first whorl bends. 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, Cretaceous and 
Tertiary, as well as in the existing species of Nautilus early in the 
nepionic substage, as may be seen in the drawings of Henry Brooks 
ee) er 
In tracing out the distinct phyla to which different nautilian forms 
belong, it can be shown that the impressed zone is invariably con- 
sequent upon close coiling, never appearing in ancestral forms in 
the nepionic stage unless through this agency. Asa rule, it comes 
in the ontogeny after this stage, usually in the ananeanic substage 
of more generalized and less closely coiled shells, but when one 
ascends ‘in the same genetic series to the more specialized nautilian 
involved shells this purely acquired character becomes, through the 
action of tachygenesis, forced back, appearing as a rule in the 
nepionic stage before the whorls touch. It is therefore in these 
forms entirely independent of the mechanical cause, the pressure of 
one whorl upon another, which first originated it. One need 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. I have so far found only one form 
—Cranoceras of the Devonian—in which there is apparently a 
slight dorsal impression, which may have arisen independently of 
close coiling. 
There are apparent exceptions to this rule in some of the ex- 
tremely close-coiled forms of nautilian shells of the Calciferous and 
Quebec faunas (some of which are figured in the plates of this 
raemoir), but in these the first whorl bends so abruptly and enlarges 
with such extreme rapidity that the inflection of the dorsal side 
before the whorls touch can be attributed to mechanical effects of 
