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The American Geologist. November, 1898 
agerontic substage is not. in my opinion, "atavistic" or "rever- 
sionary," as it is definedby Buckman and Bather. Reversions 
are the returns or recurrence of ancestral characteristics in 
genetically connected organismp which have been for a time 
Intent in intermediate forms. I do not think that we can in- 
clude in this category purely morphic characteristics which 
habitually recur in the same individual as the result of para- 
plasis, or which occur in the paracme of a type more or less 
invariably. In the individual the smooth round shell of the 
whorl of the paragerontic substage after it has lost the progres- 
sive characteristic of the ephebic stage cannot be considered 
as reversions. They aresimply analogies in form, not structur- 
ally similar characteristics. A better known and more easily 
understood case is the resemblance of the lower jaw of the 
infant before it has acquired teeth, and that of the extremely 
old human subject in which these parts have been lost and the 
alveoli and the upper parts of the bony mandible have dis- 
appeared through resorption. The forms are similar, hut no 
one would venture to consider the infant's cartilaginous jaw 
and that of the old man as similar in structure. 
The best example of similar phenomena in the phylum 
known to me is the close resemblance of form between the 
straight Baculites of the Cretaceous or Jura and Orthoceras 
of the Paleozoic. These two are often confounded by those 
ignorant of the essential differences existing in their struc- 
ture. One is a Mesozoic straight form derived by degenera- 
tive processes of evolution from the highly ornamented pro- 
gressive Ammonitinre of the Mesozoic, and the other is a near 
relative of the primitive ancestral forms of the nautiloids in 
the Paleozoic. One occurs in the paracme and the other in 
the early epacme of the group of chambered shells. They 
are widely distinct iii their structural characteristics and 
these differences are greater in the young than at any subse- 
quent stage of their ontogeny. Baculites has a close coiled 
>hell iii the nepionic period, as has beenlately demonstrated 
by Amos P. Brown of Philadelphia, and Orthoceras is straight 
from the earliest stage. The return of a similar form in 
Baculites in the later periods of development in obedience 
b0 the law of the cycle does not carry the structure back with 
it to a repetition of the orthoceran siphuncle and sutures. 
