. The Geology of Minnesota . l is developed for at least a portion of its length, as it is also in Nanno and Gameroceras. In the two genera last named the median and later portions of thesipho are constituted of deflected portions of the septa known as the siphonalfunnels. These often extend from one septum to, or beyond the one preceding it,thus separating the siphonal cavity from that of the air-chambers; but not infre-quently these funnels do not completely cross the air-chambers. In either casethese funnels form a discontinuous siphonal wall. In Piloceras, Gameroceras andNanno the continuo
. The Geology of Minnesota . l is developed for at least a portion of its length, as it is also in Nanno and Gameroceras. In the two genera last named the median and later portions of thesipho are constituted of deflected portions of the septa known as the siphonalfunnels. These often extend from one septum to, or beyond the one preceding it,thus separating the siphonal cavity from that of the air-chambers; but not infre-quently these funnels do not completely cross the air-chambers. In either casethese funnels form a discontinuous siphonal wall. In Piloceras, Gameroceras andNanno the continuous apical portion of the sipho is thickened by the extravagant depo-sition of testaceous matter in the cavity or between the successive siphonal sheaths. In 766 THE PALEONTOLOGY OF MINNESOTA. [General characters of cephalopods. Nanno this solidified portion of the sipho protrudes behind the septate portion ofthe shell, indicating a primitive condition in which the inhabited shell was a simpleaseptate and asiphonate cone. 5f:. Fig. 7.— outer shell or conch; s. septa; sf. siphonal funnels; sh. solid apical portion or guard of sipho. In certain more advanced types, Actinoceras and Gonioceras, where thesiphonal tube is continuous throughout its extent, as in Orthoceras, the sipho takesthe form of a succession of beads expanding into the air-chambers and contractedwhere meeting the septa. Within the siphonal tube is a thick deposit of shellymatter, leaving a narrow central passage or endosiphon, wlych may be completelyclosed in the earlier siphonal beads and quite wide in those of the last thickening of the siphonal wall is, thus, to some extent, a process common toall lime-secreting organisms, tending to fill up and close deserted cavities. Fi-omthe endosiphon of Actinoceras are given off series of radiating filaments penetratingthe thickened walls and reaching the true siphonal tube. In the following pages about fifty species of these fo
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