772 THE PALEONTOLOGY OF MINNESOTA. 
LNanno aulema. 
Another and more slender specimen measures 40 mm. in length and is broken at the 
aperture. Here the length of the apical cone is 22 mm. The most complete exam- 
ple has a length of 58 mm.; the apical cone measures 15 mm.; the entire diameter 
of the shell is 15 mm. at its widest part and 16 mm. at or near the aperture. 
Dr. Holm’s species, #. (Nanno) belemnitiforme, is considerably larger than N. 
aulema. The author’s figures shows that the siphonal cavity may be entirely filled 
with crystalline calcite while the air-chambers contain only the mud of the matrix. 
This is a mode of preservation which we find to be not infrequent in forms of true 
Endoceras or Cameroceras. Others of these figures (Plate 1, figs. 2a, b) show the actual 
thickness of the true calcareous wall of the przseptal cone, and indicate that it is 
considerably thinner than in N. aulema. Figure 1b shows that the wall of the conch 
becomes thinner toward the posterior cone and actually disappears upon the surface 
of the latter, though we are justified in the assumption, supported by the slight 
evidence afforded by the Minnesota shells, that the true conch was represented by a 
tenuous layer over the proximal surface of this cone. In N. belemnitiforme the siphonal 
funnels are seen to extend each the length of two air-chambers. Notwithstanding 
the reference by the Swedish author of such shells to the genus Hndoceras, we 
believe it to be proper and necessary to remove them from that association. Were 
the initial parts of the abundant forms of Endoceras (Cameroceras) constituted of 
such solid cones, they would be the portions of the shell most readily preserved; 
just as in Nanno aulema the siphonal cones are the parts almost exclusively met with. 
But no such bodies are known except in these two species. Our own observations 
upon Hndoceras lead us to the belief that the thickened posterior end of the sipho in 
that genus was nearly, if not wholly, enclosed by the chambered shell; and this impres- 
sion is in accordance with Holm’s statement that a specimen of Hndoceras burchardi, 
with a posterior diameter of but a few millimeters, was already septate. The continu- 
ance of an aseptate condition for a considerable period in the early history of Nanno 
is of itself indicative of an important difference from Hndoceras (Cameroceras) and 
Piloceras, inasmuch as this determines it to have been a more elementary organism 
than either. Of the initial parts of Piloceras little or nothing is known, but with 
what we are justified in assuming in regard to the early conditions in both Camero- 
ceras and Piloceras, and with what we know concerning Nanno, the last presents to 
us the simplest known type of cephalopod structure. 
In these shells we have before our eyes the abrupt change from a simple conical 
cavity, which was not only a potential sipho but an actual chamber of habitation, to 
a septate conch with an actual sipho continuous with the primitive habitation 
chamber. Holm has expressed in an interesting manner the course of the modifica- 
