210 OSTRACEA. 



larger valve with a transverse, basal, smooth area, with a triangular 



pit." 0. basalis, fig. 207. 

 ORTHOCERA. Lam. See Nodosaria. 

 ORTHOCERATA. Lam. A family of Polythalamous Cephalopoda, 



Lam. containing the following genera: — 



1. Conularia. Conical, externally striated; no siphon. 



Fig. 469. 



2. Amplexus. Cylindrical ; margins of the septa reflected. 



Fig. 463. 



3. Orthoceratites. Straight, gradually conical ;- septa sim- 



ple ; siphon central. Fig. 464. 



4. Nodosaria. Divided externally into lobes. Fig. 465. 



5. Belemnites. Straight, conical; septa simple; siphon 



lateral ; apex solid ; internal cast, or nucleus, pyramidal, 

 separable. Fig. 466 to 468. 



6. Conilites. Like Belemnites, but external shell thin at 



the apex. Fig. 470. 



ORTHOCERATA. Bl. The first family of Polythalamacea, Bl. 

 containing the genera Belemnites, Conularia, Conilites, Orthoceras 

 and Baculites. De Blainville remarks that the genera included in 

 this family are all fossils, and known very imperfectly, in conse- 

 quence of the greater part of the specimens being only casts. 



ORTHOCERATITES. Auct. Fam. Orthocerata, Lam. and Bl.— 

 Descr. Straight, conical, divided into numerous chambers by 

 simple septa perforated by a central siphon. 0. annulata, fig. 464. 



OSTEODESMA. Desh. Periploma, Schum. 



OSTRACEA. (Ostracees, Lam.) A family belonging to the second 

 section of the order Conchifera Monomyaria, the shells of which 

 are described as irregular, foliaceous, sometimes papyraceous, 

 with the ligament wholly or partly interior. The principal differ- 

 ence between the Ostracea and the Pectinides consists in the 

 absence of the auricles and the foliated structure of the shells, 

 for, although the Spondylus has ex-foliations or spines upon the 

 external surface, the shell itself is compact and firm. This family 

 contains the genera Gryphsea, Ostrea, Vulsella, Placuna, Anomia, 

 which may be thus distinguished : — 



