1893.] Phylogeny of an Acquired Characteristic. 875 
tion or phylogeny of the impressed zone; and all the correlative 
characteristics which accompany the inheritance of the im- 
pressed zone in the cyrtoceran stages of species which have 
this peculiarity have been observed to be present. 
The terminal members of the Nautiloids are, of course, the 
existing species. Nautilus pompilius has been examined in a 
considerable number of specimens and in all of these the 
‘impressed zone and correlative helmet-shaped whorl and 
broad flattened dorsal side appears during the cyrtoceran 
stage. Pl. XVIII, figs. 15-16 are outlines of the shell of this 
species during the cyrtoceran stage exhibiting the helmet 
shaped whorl, broad dorsum, or inner side, and its impressed 
zone, iz. Thus, when the whorls touch, as in all the nautilian 
shells of the Carboniferous, Jura and Cretaceous in which the 
same acceleration of development also occurs, the whorl is 
already prepared to become involute and to mould itself more 
readily and rapidly over the surfaces of the apex and the side 
of the succeeding whorls. In other words, heredity has begun 
the work before the whorls touch, and before the deepening 
and enlargment of the impressed zone through the pressure 
of close coiling is begun. There are quite a number of char- 
acteristics in the species of existing Nautili which lead to the 
inference that they are survivors of Jurassic and generalized 
Cretaceous and Cenozoic forms; the size of the umbilical 
perforations, the smoothness of the shells, the simplicity of the 
sutures, and soon. These facts are of importance only in 
so far as they show that the existing Nautilus does not repre- 
sent the acme of progress of its order but is a descendant of 
shells with less complicated structures than many of the genera 
of the Carboniferous, Jura, and Cretaceous. 
EXPLANATION OF PLATE XVIII. 
LETTERING. 
a. Apex of shell. This usually bears a scar on the point, 
as shown in figs. 14 and 15, but this has no bearing on the 
question discussed, and has not been described. This also 
represents the youngest or cyrtoceran stage in the growth of 
the shell, fig. 8 being a young shell with complete living 
