GENERA OF SHELLS. 107 



nearly semicircular ; parietes articulated by 

 sinuous, branch-like, indented sutures. Septa 

 transverse, sinuous, lobed and indented at the 

 circumference. Tube or siphon marginal, not 

 perforating the septa. 



TURRILITES. Shell spiral, turreted, multi- 

 locular, whorls contiguous, all visible ; parietes 

 articulated by sinuous sutures. Septa trans- 

 verse, lobed and indented at the circumference. 

 Aperture rounded. 



The turrilites, instead of being discoidal, or simply 

 arched, are elongated, straight, and form a very ele- 

 vated spiral, which it seems, must terminate in a point 

 like the turritella. 



BACULITES. Shell straight, cylindrical, 

 sometimes slightly compressed, rather conical; 

 parietes articulated by sinuous sutures. Septa 

 transverse, near together ; disc of the septa 

 imperforate, lobed and indented at the circum- 

 ference. 



The chambers of these shells are transverse and 

 narrower than those of the turrilites. 



SECT. II. MONOTHALAMOUS CEPHALOPODA. 



Shell unilocular, wholly external and envelojring the animal. 



ARGONAUTA. Shell univalve, unilocular, 

 involute, very thin ; spire bicarinate, tubercu- 

 lous, re-entering the aperture. 



A. argo, tuberculosa, nitida. 



