PUBLICATIONS. 865 
However as one ascends in the same genetic series to the more spe- 
cialized nautilian involute shells, this purely acquired character through 
the action of the law of acceleration or tachygenesis becomes forced 
back, appearing as a rule in the nepionic stage before the whorls come 
in contact. 
Chapter IV. is devoted to descriptive terms used in the following 
chapter, in which a large number of genera and species are described, 
some of which are new, with special reference to the history of the 
impressed zone. 
The last chapter is a summary, and from it may be quoted the fol- 
lowing conclusions which seem to be justified by the facts and argu- 
ments brought forward: 
“7. The impressed zone is primitively a contact furrow, an acquired 
characteristic of the dorsum of the whorls of nautilian shells having 
large umbilical perforations, which appear either in the ananeanic or 
metaneanic substages, and rarely later in their ontogeny. There is 
abundant positive evidence that in these primitive forms this furrow is 
purely a mechanical result of the nautilian mode of growth, not 
appearing in the ontogeny before. contact and either partially or 
entirely disappearing on the free gerontic volution. 
“2. The impressed zone does occur independently of contact on 
the free Jorg of ene ee substage as a Loreal furrow in 
group and in the Devon 
“3. While there is no positive proof that the dorsal furrow origi- 
nated through heredity in the paranepionic substages of these nauti- 
loids of pre-Carboniferous age, there is also no satisfactory evidence 
that it originated in the young of such species as have this character 
through purely mechanical agencies. 
“‘4. There is no positive evidence that the similar dorsal furrow 
which also appears at the same age in the young shells of Coloceras 
globatum and perhaps Coelogasteroceras canaliculatum among Carbonif- 
erous nautiloids can be explained only when it is considered as a 
transmitted, tachygenetic characteristic. 
“5. This fourth conclusion is supported by the presence of a simi- 
lar dorsal furrow in the paranepionic substage of the young shells of 
all the nautiloids of the Jura so far as observed. 
“6. The fourth and fifth conclusions are rendered still more prob- 
able by the presence of the dorsal furrow at an earlier age, the meta- 
nepionic substage, in all the nautiloids so far as observed, from the 
