874 The American Naturalist. [October, 
this result is also directly connected with the observed fact of 
the quicker development of the coiling up tendency in the 
young of these Jurassic shells. This is shown by the small 
diameter of the umbilical perforation in the centers of the 
shells of the Carboniferous. It is also connected with the fact 
that the primitive uncoiled forms, orthoceran, cyrtoceran and 
gyroceran shells begin to die out in the Carboniferous and 
cease with the Trias as mentioned above. 
This demonstration of the characters that accompany pro- 
gress in close coiling, enables me to fill a gap which occurs in the 
evidence during the Cretaceous. In this period the existence 
of the impressed zone during the cyrtoceran stage of individ- 
uals has not been clearly established by observation except in 
two species, a form allied to Cymatoceras pseudoelegans 
D'Orbigny, from Faxoe, and Cymatoceras elegans from Rouen. 
In other shells, although a considerable number have been 
broken down, the state of preservation has been invariably 
imperfect. The coiling, however, in the young of all the 
shells examined is notably more accelerated than in the simi- 
lar shells of the Jura, and the whorls broader and having 
more specialized characteristics correlative with closer coiling 
and the early existence of an impressed zone. It is, therefore, 
-fair to infer that the evidence when accessible will confirm the 
facts observed in previous periods. 
The same arguments apply also to the Cenozoies, except 
that in this period there is as yet no evidence of the early 
inheritance of the impressed zone. I have not yet succeeded 
with the Aturia, which is the only genus represented by favor- 
ably preserved specimens within my reach, in exposing the 
apex of tke whorl. The shells of this period, so far as I know 
them, are, however, excessively involute and have exceedingly 
.small umbilical perforations with very deep impressed zones 
after the whorls touch. The umbilical perforation in Aturia 
is in fact smaller than in any nautiloid known to me. 
The imperfect evidence so far gathered in the Cretaceous, 
and the absence of positive evidence in the Cenozoics, does 
not, therefore, seriously affect conclusions reached in this 
paper, since these are merely gaps in the history of the evolu- 
