768 



THE PALEONTOLOGY OF MINNESOTA. 



[Plloceras newton-winehelli. 



ridge or annulus on the outer surface of the sipho. The latter tube is, hence, a 



es 



Fig. 8. Median vertical section of one of the 

 specimens described, x 2 



Fig. 9. Enlargement of a portion of the same. 

 s. sipho; ss. siphonal sheaths; ws. wall of sipho; 

 es. endosiphon; sp. septa; a. annulus formed at 

 junction of septum with sipho. 



distinct sheath of itself, not originating from successive invaginations of the septa. 

 The earlier portion of the sipho (s.) contains two siphonal sheaths (ss.) which are 

 separated from each other by an empty space, and above the first of these sheaths 

 there is also but a partial and irregular filling of silica. Below the second sheath 

 the sipho is solidly filled. These sheaths take their origin from thickened walls of 

 the sipho, the separation of the testaceous wall being very apparent at their origin. 

 When the sheaths become fully free from the siphonal walls, they converge more 

 or less rapidly. The first or uppermost of the sheaths is very plainly perforated at 

 its apex, but this opening is less clearly retained on the lower sheath. These open- 

 ings are undoubtedly to be construed as the passages of the endosiphon, as Hyatt 

 has termed a small tube connecting successive deposits and compartments within 

 the sipho. In the larger forms of Piloceras which have been described by Dawson 

 and Whitfield (P. amplum, P. explanator) and whose siphones have become solid from 

 the more rapid formation of these sheaths and the intermediate deposition of crys- 

 talline calcite, this endosiphon is sometimes very clearly retained,* but here where 

 the chambers of the large cavity are open, nothing remains but the passages of the 

 endosiphon through the sheaths. In this respect the structural difference is note- 

 worthy as the case in hand is the only one observed in which the siphonal chambers 



See Dawson, Canadian Naturalist, new series, vnl. \. Nn. 1, 1881, p. 1-4; and Foord, Geological Magazlnn, Dec. Ill, vol. 

 Iv, No. 12, 1887. 



