1893.] Phylogeny of an Acquired Characteristic. 869 
posed to be perfect and the internal structures concealed.* The 
young of pl. XVHI, fig.4, which represents the fourth class of 
forms repeats the cyrtoceran form, then curves more closely and 
just before it comes in contact there is a short time when it over- 
laps the apex without touching it. At this time it is plainly gyro- 
ceran like the whole of fig. 3. After it touches the first whorl just 
beyond the apex it remains in contact and the inner side or 
dorsum of the second or overlapping whorl begins to show a 
flattening as a result of this collision of the whorls. The sec- 
tions of the orthoceran, cyrtoceran and gyroceran whorls 
show no such flattening in any of the specimens examined, 
although hundreds of different kinds have been studied. The 
sections are designated on the plate by the same letters as the 
supposed lines of the sections made through the tube, and 
although diagrammatic figures, they give a sufficiently clear 
general explanation of the facts observed. More specific fig- 
ures could have been given in abundance and will be given 
in the paper now in course of preparation. 
Pl. XVIIL, fig. 5, shows the same phenomenaas figure 4. The 
young is at first cyrtoceran like the adult whorl of figure 2, 
and open, then becomes gyroceran in curvature and finally 
overlaps the apex when it has arrived at the end of the first 
volution, but does not at first touch it. Then coming into 
contact it acquires a flattened area or faint impressed zone on 
the dorsum or inner side of the second volution as is shown in 
the section fig. 5c. This is similar to the section of figure 4 
shown in fig. 4c’, which represents a cut through an adult 
whorl of the fourth class of forms. It differs only in being 
smaller on account of the younger stage of growth at which 
it occurs. 
The entire series of forms from orthoceran to nautilian is 
more or less represented, even in the earliest period at which 
the Nautiloids appear, namely, in the rocks of the Quebec 
Group. There is, however, this qualification: the fifth class 
of forms, or the involute nautilian, are comparatively rare 
and become more abundant in successive periods. The young 
2 Except in fig. 9 in which a portion of the shell is broken away showing the 
cast of the interior andthe sutures. 
