314 
velopment in that part of the organism that was most vitally affected by 
the environment, and which must therefore indicate most perfectly the 
lines along which the evolution proceeded. 
If the initial portion of the shell of Nawtilus be examined, it will be 
found to be characterized by a scar or cicatrix. In the same region of 
the shells of ammonites and some Nautiloidea (Orthoceras), instead of this 
cicatrix, there is present a minute, bulbous or bag-like shell, attached to 
the apex of the shell proper. If in the case of Orthoceras, as shown by 
Hyatt (31), this bulb, or protoconch be broken away, there is exposed a 
sear (cicatrix) precisely similar to that of Nautilus. The initial shell or 
protoconch is therefore substantially the same in all of the Tetrabranch- 
iata, and is supposed to point to a “‘septa-less and chamberless form simi- 
lar to the protoconch” as the common ancestor of these two great divisions 
of the Tetrabranchiata; and possibly, as Hyatt suggests of the Cephalo- 
poda, Pteropoda and Gastropoda (31). The protoconch represents the 
latest of the true embryonic stages, namely the phylembryo. 
Succeeding this early stage are the stages of the shell proper.’ In 
Nautilus the early nepionic portion of the shell, which includes the forma- 
tion of the first three septa, is only slightly curved (cyrtoceraform). Up 
to the stage of the formation of the second septum, the shell is in fact 
nearly straight (orthorceraform). The first septum has an apically di- 
rected caecum, and the second septum an apically directed closed tube, 
the closed apical end of which fits into the caecum of the first septum. 
This tube is the beginning of the siphuncle. Since the tube fits closely into 
the caecum, the two together form a continuous tube, in which the apical 
end or bottom of the siphuncular tube forms a partition or septum, so 
that as Hyatt points out, the resemblance “of this early stage to the adult 
structures of Diphragmoccras becomes perfectly clear.” (31) 
In the later nepionic stages (i. e., after the formation of the third 
septum) the shell is rather sharply bent (the gyroceran curve), so that 
near the close of the first volution the whorl is brought back into contact 
with the apex of the conch. This manner of growth results in leaving 
an empty space or umbilical perforation between the two halves of the 
first volution. In the ancient coiled Nautiloidea there appears at the be- 
ginning of this (neanic) stage, when the whorls come into contact, a de- 
1The stages from this point on are termed by Hyatt (31), and following him 
by practically all paleobiologists at the present time, the nepionic, neanic, ephebic 
and gerontic stages; meaning respectively, the infantile, youthful, mature and old 
age stages of growth. 
