40 



NORTH AMERICAN INDEX FOSSILS. 



tube continued in endosiphocoleon in adult shell; apical portion 

 long, without camerse, but with endosiphosheaths. 



Very common, Fort Cassin (Beekmantown) beds of Lake Cham- 

 plain region. 

 3. C. tenuiseptum (Hall). (Fig. 1235.) Ordovicic. 



Fig. 1234. Protocaine- 

 roceras brainerdi, lateral and 

 end views, X ^- (After 

 Whitfield, Am. Mus. Nat. 

 Hist. Bull.) 



Fig. 1235. Cavieroceras tenuisephim, specimen showing 

 part of shell, and natural section, showing endosiphotube, 

 X%- (After Ruedemann, Bull. 90, N. Y. State Mus.) 



Differs from preceding in its more rapidly tapering shell (13 mm. 

 in 100) ; circular section; large siphuncle which occupies one half 

 the diameter of the adult ; more closely crowded and very deep and 

 exceedingly thin septa, with depth three times that of the camerse ; 

 suture with low ventral saddle. 



