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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 



(H. i). 



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. As a 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 

 memoir), 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 



