772 THE PALEONTOLOGY OF MINNESOTA. 



I Viiiino 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 IS mm. at its widest part and 16 mm. at or near the aperture. 



Dr. Holm's species, E. (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 i, figs. 2a, b) show the actual 

 thickness of the true calcareous wall of the praeseptal cone, and indicate that it is 

 considerably thinner than in N. aulema. Figure Ib 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 Endoceras, 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 Endoceras 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 Endoceras 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 Endoceras (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- 



