776 THE PALEONTOLOGY OF MINNESOTA. 



[OMneroceras. 



retrally directed funnels of the septa. This is well illustrated by a very large and 

 essentially entire internal cast of the sipho, measuring 3 feet 3 inches in length, a 

 reduced drawing of which is shown on plate XLVII. The principal extent of the 

 surface of this specimen bears traces of the contiguous or overlapping septal funnels 

 which have thus created a discontinuous siphonal tube. The specimen is so drawn 

 as to show the side nearest the margin of the conch; hence the remnants of the septa 

 show a marked angularity due to their concavity. This is a trait characterizing all such 

 siphonal casts in which the position of the sipho was lateral. With variation in 

 the position of the sipho and in the concavity or obliquity of the septa, these siphonal 

 funnels vary in their direction. It will be observed that at a short distance from 

 the apex of this cast is an abrupt contraction in its diameter, which is continued to 

 the apex. This apical portion is the internal filling of the solid and continuous sheath 

 whose probable extent and thickness is indicated by a dotted line which carries the 

 sipho to an apex according to the slope of the discontinuous sheath. The evidence 

 afforded by this specimen is abundantly fortified by others which demonstrate that 

 this solid apical portion of the sipho is but a filling and thickening of the vacated 

 and discarded apical cavity of the otherwise discontinuous sheath. 



The internal casts of this long apical cone are of far more frequent occurrence 

 than those of the funnel-tube, because the parts of the latter are not often coherent 

 and usually the latter are found to retain the overlapping or approximate parts of 

 the septa upon their surface. 



The great siphones of the dead shells of these creatures afforded favorite retreats 

 for other and smaller cephalopods, and they are hence frequently found crowded 

 with diverse species of Orthoceras and Cyrtoceras, sometimes three or four being 

 crowded in side by side, or one within another, in such cavities. 



The material examined does not afford the most satisfactory evidence of the 

 duplication of the siphonal sheath, and it would seem that much of the evidence that 

 has been adduced in regard to the presence of such duplicate sheaths requires 

 re-examination, although it is by no means intended by this expression to cast doubt 

 upon their existence. In some accounts of these fossils a careful distinction between 

 the apical sheath, its filling, and such adventitious or hermit orthocerans as may 

 have got in, has not always been made. Attention may be directed to the internal 

 siphonal cast shown on plate L, fig. 3, which shows a portion of the filling of the 

 discontinuous part of the tube and an acuminate terminal process which indicates 

 a tubular cavity near the apex of the solid sheath. Whether or not this ever 

 penetrated the solid sheath and thus represents a true endosiphon communicating 

 with some other sheath, as in Piloceras, cannot be determined from such casts. 



