560 
tions, making a complete series of all of the substages of develop- 
ment in Wautilus pompilius and more or less of other species, and 
also to Dr. R. T. Jackson, for similar material. I hope to use this 
material more extensively and effectually in the future. In this 
paper full justice cannot be done to the work of Dr. Beecher or 
Mr. Brooks. 
This generic name, heretofore supposed to include nearly all 
of the coiled or nautilian forms of Nautiloidea and still used by 
some conservative paleontologists in this way, is really not applica- 
ble to any forms except the living species of nautiloids and possi- 
bly some shells inthe Tertiary. Even these last cannot be satisfac- 
torily referred to the genus Nautilus until their nepionic substages 
have been worked out. 
The genus Eutrephoceras is a near ally but still distinct in most 
of its characteristics. The broad outline of all of the epinepionic 
' stages of growth, the general position of the siphuncle, dorsad of 
the centre, and the distinct sutures of Eutrephoceras separate the 
species. The minute umbilical perforations and closer coiling of 
the younger substages of the conch in Eutrephoceras show also that 
it is the terminal group of some other genetic series than that to 
which Nautilus probably belongs. 
The genus Cymatoceras of the Cretaceous differs in the broad 
costations as well as in the outline of the nepionic whorl. The 
sutures of this genus are more like those of Nautilus than the 
sutures of Eutrephoceras. 
The genus Nautilus is obviously still more remote from Cenoce- 
ras of the Jura in the sutures of all stages and form of the ananepi- 
onic and succeeding nepionic substages, although in the outline of 
the ephebic whorl and position of the siphuncle there is close 
approximation. If one excepts the comparison of the ananepionic 
substage, which is obviously similar to that of Huérephoceras Dekayt, 
being only more compressed, the nepionic stage and the ananeanic 
substage are very close in aspect to those of Digonioceras, although 
the succeeding substages become quite distinct. 
I cannot in this memoir give full descriptions of the substages of 
development which I hope to treat fully in the future. It will suf- 
fice to refer to the accurate drawings of Mr. Brooks, given on PI. i, 
and to notice the fact that young shells and preparations now in 
my possession of Wautilus umbilicatus, pompilius and macromphalus 
show no variations in their characteristics worth noticing here. It 
