876 The American Naturalist. [October, 
chamber. This letter also indicates the location of the sec- 
tions correspondingly lettered in the figures. 
b is used to indicate the section of the cyrtoceran stage in 
figs. 11-13. 
b’ is used to indicate the place of the sections, figs. 4—5b’, 
upon the whorls of figs. 4-5. They were taken through the 
whorl in the gyroceran stage. 
c is used for the adolescent stage of growth in the whorl 
and the corresponding sections. 
ce’ is used for the full grown stage in the growth of the 
whorl and the corrésponding sections : 
d for the first part of the senile stage: 
efor the final and most degenerative part of the. senile 
stage: 
i. z. for the impressed zone. 
v venter or outer side of the shell, the dorsum being the 
inner side of the whorl. 
w for the whorls, thus 1 w in figs. 3 and 4 means the end of 
the first whorl, 2 w the beginning of the second whorl, 3 w 
that of the third whorl. These letters serve to show the 
progressive increase in numbers of the whorls in the different 
classes of forms. 
FIGURES. 
Fig. 1. Outline of an orthoceran shell. 
Fig. 2. Outline of cyrtoceran shell. 
Fig. 3. Outline of gyroceran shell. 
Fig. 3. Outline of nautilian shell, having a larger umbilical 
perforation at (a) and fewer whorls at the same age, than in 
fig. 5, in other words it is less tightly and completely coiled 
up than the class of shells represented by that figure. 
Fig. 5. A nautilian shell with tighter coils than in fig. 4 and 
the whorls coming in contact and the impressed zone begin- 
ing at an earlier stage. 
Fig. 6. Barrandioceras———(sp. Barrande) Hyatt, showing 
the most involute of the Silurian shells so far as known ; fig. 6 
is reduced in size but the section fig. 7 is natural size. 
