224 OSTEACEA, 



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



Kg. 469. 



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



rig. 463. 



3. Obthoceratites. Straight, gradually conical ; septa 



simple ; siphon central. Fig. 464. 



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



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



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

 separable. Eig. 466 to 468. 



6. CoKiLiTES. Like Belemnitea, but external shell thin at 



the apex. Fig. 470. 



OETHOCERATA. Bl. The first famUy of Polythalamacea, Bl. 

 containing the genera Belemnites, Conularia, Conilites, Ortho- 

 ceras and Baculites. De Blainville remarks that the genera 

 included in this family are all fossils, and known very imper- 

 fectly, in consequence of the greater number of the specimens 

 being only casts. 



OETHOCEEATITES. Auct. Fam. Orthocerata, Lam. and Bl. 

 — Descr. Straight, conical, divided into numerous chambers by 

 simple septa perforated by a central siphon. O. annulata, 

 PI. xxii. fig. 464. 



OSTEODESMA. Desh. Peeiploma, Schum. 



OSTEACEA. {Ostracees, Lam.) A family belonging to the 

 second section of the order Conchifera Monomyaria, the shells 

 of which are described as irregular, foliaceous, sometimes papy- 

 raceous, with the ligament wholly or partly interior. The prin- 

 cipal difference 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 contaias the genera Gryphsea, Ostrea, Vulsella, 

 Placuna, Anomia, which may be thus distinguished : — 



1. Pedum. Elat, turned up at the sides, an hiatus for the 

 passage of a byssus.. A triangular disc on the hinge. 

 Pig. 179. 



