NATICID^E. 9 



Subgenus GYRO DBS, Conrad, 1860. 



Shell depressed globose ; aperture generally angular or nar- 

 rowly rounded below ; inner lip thin ; umbilicus wide, docp, 

 without callosity, bounded by a revolving carina which is some- 

 times crenate, with occasionally a second small revolving ridge 

 within ; whorls shouldered above, the angle generally wrinkled 

 or crenate. 



Cretaceous, United States, India. N. alveata, Conr. (S. and S. 

 Conch., t. Ixiv, f. 70). 



Subgenus TYCIIONIA, de Koninck, 1881. 



Shell somewhat depressed, globose, smooth; spire short, 

 obtuse, suture linear ; last whorl very large, depressed at the 

 base; aperture semilunar ; lip thin; colnmella thickened by a 

 callosity which is limited by a shallow oblique groove ; no 

 umbilicus. 



N. Omaliana, de Koninck. Carboniferous, Belgium. 



y Subgenus PLATYOSTOMA, Conrad. 



Shell subglobose ; spire short ; aperture very large, suborbicu- 

 lar, dilated ; labrum joining the body-whorl at right-angles to the 

 axis of the shell. 



P. Niagarensis, Hall. Niagara group, New York (S. and S. 

 Conch , t. Ixiv, f. 74). 



Section STROPHOSTYLUS, Hall. 



Shell subglobose, spire small, body-whorl large, ventricose ; 

 outer lip thin, sometimes slightly expanded ; columella twisted 

 or spirally grooved within, not reflected; umbilicus none. P. 

 subobtusa, Hall. Lower Helderberg, N. York (S. and S. Conch., 

 t. Ixiv, f. 75). 



Differs in its twisted or grooved columella. 



Section ORIOSTOMA, Munier-Chalmas, 1876. 



Umbilicus moderate, circumscribed by a carina ; whorls some- 

 times partially free. P. Barrandei, Mun.-Chalm. Devonian. 



The relations of this group with Natica are somewhat obscure, 

 as are also those of the typical form with those designated here 

 as sections. As in many other cases with the older fossil forms 

 we can only be guided by rather remote general resemblances. 



Tylostoma, Sharpe, is considered a member of the group by 



