208 The American Geologist. October, 1894 
with noteworthy differences. The initial parts of the shell of 
Piloceras arc still undescribed, but we may assume that were 
it possessed of such a solid apical sipho as is tfanno, that 
would be the part most readily preserved, as in this case. 
The sheathing of the sipho. its great size and its relation to 
the septa arc structural points allying these two genera. The 
presence of the apical siphonal cone may indicate the exist- 
ence of X<nni<> for a considerable period in the young state 
simply as an aseptate body, and with such evidence the genus 
would seem to be a degree more elemental than our present 
knowledge ascribes to PiLocercvs, inasmuch as the septate con- 
dition in the latter genus is of earlier occurrence. It may be 
suggested that the solidification of the prseseptal cone is to 
some extent due to such secondary causes as have produced 
the solid guard in Belemnites. The appearance of these 
siphones and the crystalline structure of their substance 
st rongly suggests that genus, even though there is little super- 
ficial similarity in the relations of these parts to the sep- 
tate portions of the shell in the two genera. 
Geological horizon. The material studied consists of seven 
specimens obtained from the Trenton shales of Minneapolis 
and from the Galena shales at Chatfield, Minnesota. 
Explanation of Plate VI. 
Nanno aiilema. 
Fig. 1. The m<»si complete example observed; showing the form of 
the entire shell, the apical cone, posh ion of sipho and some of the septa. 
X i.3. 
Fig. 2. The praeseptal cone with a portion of the annulated siphonal 
tube: a lateral view. X 1.3. 
Fig. 3. A fragment of a sipho, showing only the cylindrical portion 
to Which a part of the smooth external shell adheres. X 1.3. 
Fig. 4. A cross-sect inn of the apical coin': showing al a the Ailing of 
the internal cavity, at b the line of subdivision of the sipho into siphonal 
sheaths. The radial Structure of the substance of the body is also indi- 
cated. X "-Mi. 
Fig. •">. Lateral view of a sipho as usually found, with smooth cone 
and cylinder. X 1.3. 
Figs. <> and ",. Ventral (anti-sipho'nal) and dorsal (siphonal) aspects of 
t he same specimen. 
Fig. 8. A vertical longitudinal section of a sipho, showing at a the 
form of the internal cavity, and at 6 the line of division between the 
sheaths, x 1.3. 
