GONIATITES. 



69 



have so inucli in commou that it is very hard to distinguish them; it is 

 quite possible that they are only varieties of G. sphcencus. 



Tlie form is globose, broad, low-whorled, with semilunular cross sectiou. 

 The umbihcus is very narrow, about one-tenth of the diameter of the shell, 

 so that the inner coils are concealed. There are four or five constrictions 

 to a revolution, visible both on the shell and the cast. The whorl is 

 exceedingly involute, each whorl being indented to about three-fifths of its 

 height l)y the 2Ji'eceding-. The shell is marked with distinct cross striae with 

 fine, sharp crenulations, which show only toward maturity. The elevations 

 between the pits of the crenulations become in the adult indistinct spiral 

 striations, giving a finely reticulate aspect to the surface. These are not 

 visible on the cast. In the adult stage the cross striai bundle near the 

 umbilicus, forming weak incipient nodes. 



From G. sphcerkus this species is distinguished by being more com- 

 pressed laterally, less globose, having narrower umbilicus, weaker spiral 

 stria', and coarser cross striae. From G. striatus this species may be distin- 

 guished chiefly by its slightly narrower und^ilicus, its finer spiral stritie, and 

 coarser cross striae. The table below, compiled from Foord and Crick's 

 catalogue, shows the principal differences and agreements between the 

 three species: 



Breadth in proportion to diameter . . 

 Height of whorl to width 



Height of whorl to diameter 



Width of umbilicus to diameter 



Indentation of last whorl by preced- 

 ing. 

 Constrictions to revolution 



Septa to revolution. 

 Spiral strise 



Cross striie 



G. crenislrLa. 



B=f D 



Wider than high . . 



f D , 



U=iD 



B=f D 



A little wider than 



high. 

 A little over ^ D . . 



U=tVD 



Indented more j Nearly § 



than J. 

 (4 in American 4 to 5 feeble 



specimens. ) 



19-20 



(Fine in American 



specimens. ) 

 Finely crenulated . . 



20 , 



Strong and sharp. . 



G. sphxriciis. 



B=|D. 



5W. 



U = J D. 

 More than J. 



4 or more faint. 



18. 



Very fine. 



Sharply incised, Obscure, 

 sinuous. 



Not enough specimens have been studied to show that these characters 

 are really constant; that they are not constant is shown by the fact that 



