44 



NORTH AMERICAN INDEX FOSSILS. 



of shell and equal in width to the interspaces; septa deeply con- 

 cave; siphuncle large, subdorsan, circular in section. 

 Trenton limestones of New York. 



Fig. 1241. Cyclendoceras annulatum^ section showing siphuncle, and septa, and 

 exterior view of fragment. The fine dark lines indicate edges of septa, X Yz- (After 

 Hall, Pal. N. Y., T.) 



VI. Nanno Clarke. 



Apical end without camerae, wholly occupied by siphuncle as in 

 Proterocameroceras , but short and swollen, the siphuncle contract- 

 ing at the camerate portion; endosiphuncle (endosiphotube) re- 

 stricted to apical end; siphuncle in absolute contact with shell in 

 camerate portion, the sutures making an apparent bend apically 

 into a ventral lobe. Ordovicic. 



