318 MOLLUSCA. 



If the whorls do not envelope each other, they are the Helicos- 



TEGA AmMONOIDa(I). 



If the whorls are elevated as in most Univalves, they are the He- 



HCOSTEGA TuKBINOIDa(2). 



Simple cells may also be strung upon a single, straight or slightly 

 curved axis, constituting the family of the Stycostega(3). 



Or they may be arranged in two alternate series, when they be- 

 come the Enallostega(4). 



VERTEBnALINA, 



Cassidclina. 



(1) M. D'Orbigny divides them into four genera: 



SOLDANIA, 



OPKllCUHIfA, 



PlANOnBULIXA, 



Planulina. 



(2) These form ten genera; 

 Truxcatclina, 

 GxnoiDixA, 

 GLoniGsniKA, 



Calcaiiixa, where is placed, among otiiers, the Nautilus Spengleri, Fich. 

 and Moll., XIV, d., I, and XV. 



ROTALIA, 



Rosalina, 



Valvulina, 



Bulimina, 



Utigeuixa, 



Clavulixa. 



(3) The Stycostega are divided by M. D'Orbigny into eight genera: the Nodo- 

 SARiA, which he subdivides into the true Nodosaria, such as the Miutilus radicu- 

 lus, L. ; — Naut.jugosus, Montag., Test. Brit., XIV, f. 4; and into Dextalixa, such 

 as the Nautilus rectus, Montag., I, cit., XIX, f. 4, 7 (the genus Reophaga, Montf., 

 I, 330); into Orthocerixa, such as the Nodosaria clavulus, Lam. , Enc/cl., pi. 466, 

 f. 3; and into Mucroxixa. 



Froxdicularia, where comes JRenulina complanata, Blainv., Malac. 



LlXGULIXA, 

 RlMULIXA, 



Vagixulixa, to which belongs the Nautilus legumen, Gm., Plane, I, f. 7; En- 

 cy., pi. 465, f. 3. 



Margixulixa, where we find the Nautilus raphanus, Gm., Soldan., It, xciv. 

 PiAxrLAKiA, such as the Nautilus crepidulus, Fich., and Moll., XIX, g, h, i. 

 Pavoxixa. 



(4) M. D'Orbigny lias five genera of Enallostegat : 



BlGEXERIXA, 



Textclaria, 



VuLTUtlXA, 



DiMOHPHIXA, 



POLTMORPHIXA, 



