iS93.] 99 [Hyatt. 
a turning round and back, is not equally good. While this is 
better than the term formerly employed, nostologic, it is longer 
and not preferable to Nostic, from voo-to's, ^ signifying a return in 
the sense of a journey back to one's home. 
This paragerontic stage is in no sense "atavistic" or "rever- 
sionary," as it is defined by Buckman and Bather. Reversions 
are the returns or recurrence of ancestral characteristics in genet- 
ically connected organisms which have been for a time latent 
in intermediate forms. I do not think that we can include in 
this category purely morphic characteristics which habitually 
recur in the same individual as the result of paraplasis, or which 
recur in the paracme of a type more or less invariably. In the 
individual the resemblance of the smooth round shell of the whorl 
of the paragerontic ammoiioid after it has lost the progressive 
characteristic of the ephebic stage cannot be considered as a rever- 
sion. It is simply analogy of form not structural similarity of 
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 upper parts of 
the bony mandible have disappeared through resorption. The 
forms are alike, but no one would venture to consider the 
infant's cartilaginous ja^v 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 tlie Pale- 
ozoic. These two are often confounded by those ignorant of the 
essential differences existing in their structure. One is a Mesozoic 
straight form derived by degenerative processes of evolution from 
the highly ornamented progressive Ammonitinae of the Mesozoic, 
1 Neither of these words has any authority for the termination "ic, ' but unless one 
can make some such "corruptions," it is often impracticable to manufacture a consis- 
tent set of terms according to the method here adopted. It is obvious that scientific 
convenience occasionally requires such heroic methods, and this seems a case in which it 
is justifiable. If the new set of terms here proposed is adopted, there will be no need 
of emploj'ing either "catabatic" or "nostic." These will then be superseded by 
"anagerontic" and "paragerontic" or by all three terms used for the substages in the 
table, if the characteristics justify their application. It was necessary, however, to 
discuss these terms because two distinct sets of names have been used for the subdivi- 
sions of the senile period. 
