766 THE PALEONTOLOGY OF MINNESOTA. 
(General characters of cephalopods. 
Nanno this solidified portion of the sipho protrudes behind the septate portion of 
the shell, indicating a primitive condition in which the inhabited shell was a simple 
aseptate and asiphonate cone. 
Fig. 7.—Cameroceras proteiforme. 
C. 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 the 
siphonal tube is continuous throughout its extent, as in Orthoceras, the sipho takes 
the form of a succession of beads expanding into the air-chambers and contracted 
where meeting the septa. Within the siphonal tube isa thick deposit of shelly 
matter, leaving a narrow central passage or endosiphon, which may be completely 
closed in the earlier siphonal beads and quite wide in those of the last air-chambers. 
The thickening of the siphonal wall is, thus, to some extent, a process common to 
all lime-secreting organisms, tending to fill up and close deserted cavities. From 
the endosiphon of Actinoceras are given off series of radiating filaments penetrating 
the thickened walls and reaching the true siphonal tube. 
In the following pages about fifty species of these fossils are identified and 
described. ‘The material which has been studied may be regarded as fairly repre- 
sentative in abundance although its usual retention in the form of internal casts 
leaves our knowledge of some parts of the species unavoidably imperfect. The ° 
identification of some of the species, especially those of Cyrtoceras and Oncoceras has 
been rendered difficult by the brevity and obscurity of the original descriptions and, 
in many instances, the absence of illustrations, but, as a careful comparison has been 
made of the material with the original specimens of such unillustrated species, 
reasonable security is felt in these identifications. 
Many of the most interesting specimens are from the collection of the late W. 
H, Scofield; others have been loaned by Dr. C. H. Robbins and Mr. E. O. Ulrich. 
