CEPHALOPODA. 275 



partitions or septa, which are pierced by a tube or " siphuncle," 

 the animal itself living in the last chamber only of the shell. 



The Cephalopoda are divided into two extremely distinct and 

 well-marked orders, termed the Dibranchiata and Tetrabran- 

 chiala. The former is characterised by the possession of two 

 gills only, and by the fact that the shell, if external (as it very 

 rarely is), is never chambered. In this order are comprised 

 the living Cuttle-fishes, Squids, and Paper Nautilus, with the 

 extinct family of the Belemnitidcz, The latter is distinguished 

 by the presence of four gills, and by the possession of an exter- 

 nal many-chambered shell This order is abundantly repre- 

 sented in past time, but has no other living representative than 

 the Pearly Nautilus alone. The following table gives the char- 

 acters and leading genera of the families of Cephalopoda : 



SYNOPSIS OF THE FAMILIES OF THE CEPHALOPODA. 

 CLASS CEPHALOPODA. 



ORDER I. DIBRANCHIATA. 



Animal with two branchiae ; not more than eight or ten arms, 

 provided with suckers ; an ink-bag Shell commonly internal and 

 rudimentary ; rarely external, but not chambered. 

 SECTION A. OCTOPODA. 



Arms eight, suckers sessile. 

 Fam. I. Argonautidce. 



Female provided with a calcareous, external, monothalamous 

 shell, secreted by the webbed extremities of two of the dorsal 

 arms. Gen. Argonauta. 

 Fam. 2. Octopodidce. 



Shell internal, rudimentary, uncalcified. No pallial fins in 

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

 SECTION B. DECAPODA. 



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

 Fam. 3. Teuthida. 



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

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

 Fam. 4. Belemnitidte. 



Shell internal, composed of a conical chambered portion 

 ( " phragmacone ") with a marginvj siphuncle, produced into a 

 horny plate or "pen," and lodged in a cylindrical fibrous 

 "guard." 111. Gen. Belemnites, Belemnitella, Belemnoteuthis. 

 Fam. 5. Sepiadtz. 



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

 nating in an imperfectly-chambered apex (" phragmacone"). 111. 

 Gen. Sepia, Beloptera, Spirulirostra. 

 Fam. 6. Spirulida. 



Shell internal, nacreous, chambered, discoidal ; the whorls 

 separate ; a ventral siphuncle. Gen. Spirula. 

 ORDER II. TETRABRANCHIATA. 



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

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



