BEEKMANTOWN AND CHAZY FORMATIONS OF CHAMPLAIN BASIN 49I 



Observations. This species is characterized as a primitive 

 Actinoceras by the confinement of the filling of the siphuncle to the 

 apical and middle portions. The thickening of the septa around the 

 siphuncle observed here occurs also in Paractinoceras can- 

 ad e n s e Whiteaves. C . b o y c i i has in the depth of the cham- 

 bers, the depressed form of the conch and the nummuloidal char- 

 acter of the siphuncle some similarity to Cyrtoceras sub- 

 turbinatum Bill, from the Lower Siluric of the Mingan islands, 

 but differs in the subcentral position of the siphuncle and greater 

 curvature of the conch. 



Cyrtactinoceras champlainense sp. now 



Pjate 34, figure 3 ; plate 36, figures 1, 2 



Description. Conch a small, but slightly arcuate cyrtocera- 

 cone, which expands moderately, the transverse diameter increasing 

 within a length of 20 mm from 9 mm to 1 5 mm in one specimen ; 

 the rate of expansion itself increasing somewhat from the nepionic 

 part of the conch to the ephebic. The largest specimen observed 

 measures 44 mm with the greater portion of the living chamber and 



Fig. 48 Fig. 49 Fig. 50 



Fig. 48 Cyrtactinoceras champlainense sp. now Transverse section 

 showing the circumsiphuncular organic deposit, the siphuncular deposit and the 

 endosiphotube. \2% 



Fig. 49 Cyrtactinoceras champlainense sp. now Longitudinal sec- 

 tion showing the extent of the organic depositions in the cameras and siphuncle ; 

 the endosiphotube in the lower part of the siphuncle and the tubuli in the 

 upper. X3 



Fig. 50 Cyrtactinoceras champlainense sp. nov. Transverse section 

 of the living chamber of the specimen reproduced on plate 36, figure 1, 2. 

 Natural size 



about 10 mm of the apical portion missing. The section is slightly 

 depressed, the outer side being slightly flattened, and the dorsoventral 

 diameter smaller by about one sixth than the transverse diameter. 



Cameras very shallow, there being 9 counted in 5 mm in the apical 

 portion and the same number in 10 mm in the ephebic portion, filled 

 more or less by organic deposits in the form of rings or heart- 



