MOLLUSCA: CEPHALOPODA. 335 



Fam. I. Argonautidit. 



Female provided with a calcareous, external, monothalamous 

 shell, secreted by the webbed extremities of the dorsal arms. 

 Gen. Argonauta. 

 Fam. 2. Octofodidce. 



Shell internal, rudimentary, uncalciBed. No pallial fins in 

 most. 111. Gen. Octopus, Tremoctopus, Eledone, Pinnoctopus. 



Section B. Decapoda. 



Arms eight, with two clavate "tentacles;" suckers pedunculated. 

 Fam. 3. Teuthidce, 



Shell an internal horny "pen" or "gladius." Fins mostly 

 terminal. 111. Gen. Loligo, Onychoteuthis, Ommastrephes. 

 Fam. 4. Belemnitidtz. 



Shell internal, composed of a conical chambered portion 

 ( " phragmacone ") with a marginal siphuncle, sometimes pro- 

 duced into a homy plate or " pen," and lodged in a cylindrical 

 fibrous "guard." 111. Gen. Belemnites, Belemnitella, Belem- 

 ■ niteuthis. 

 Fam. 5. Sepiadce,. 



Shell calcareous, consisting of a broad, laminar plate, termi- 

 nating posteriorly in an imperfectly chambered apex ("phragma- 

 cone"). 111. Gen. Sepia, Beloptera, Spirulirostra. 

 Fam. 6. SfirulidcB. 



Shell internal, nacreous, chambered, discoidal ; the whorls 

 separate ; a ventral siphuncle. Gen. Sfinila. 



Order II. Tetrabranchiata. 



Animal with four gills ; arms more than ten, without suckers ; 

 no ink-bag ; shell external, chambered, and siphuncled. 

 Fam. I. Ndutilida. 



Sutures of the shell simple ; the siphuncle central, sub-centralj 

 or near the concavity of the curved shells, simple. 

 Suh-famUy Nautilid<B proper. 



Body-chamber capacious ; aperture simple ; siphuncle 

 central or internal. 111. Gen. Nautilus, Lituites, Trocho- 

 ceras. 

 Sub-family Orthoceratidce. 



Shell straight, curved, or discoidal ; body-chamber 

 small ; aperture contracted ; siphuncle complicated. 111. 

 Gen. Orthoceras, Phragmoceras, Cyrtoceras. 

 Fam. 2. Ammonitidm. 



Shell discoidal, curved, spiral, or straight ; body-chamber 

 elongated ; aperture guarded by processes, or closed by an oper- 

 culum ; sutures angulated, lobed, or foliaceous ; siphuncle ex- 

 ternal or dorsal (on the convex side of the curved shells). 111. 

 Gen. Ammonites, Ceratites, Baculites, Turrilites, Scaphilts, Ancy- 

 loceras. 



