CEPHALOPODA. 57 



tube or " siphmicle," 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 DibrancMata and Tetra- 

 hxtncMata. 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 JSTauti- 

 lus, with the extinct family of the Belemnitidce. The latter 

 is distinguished by the presence of four gills, and by the 

 possession of an external many - chambered shell. This 

 order is abundantly represented in past time, but has no 

 other living representative than the Pearly Nautilus alone. 

 The following table gives the characters and leading genera 

 of the families of Cephalopoda : — 



Synopsis op the Families of the Cephalopodx\. 



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. 1. Argonautidce. 



Female provided with a calcareous, external, monothalamous 

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

 arms. Gen. Argonauta. 

 Fam. 2. Octopodida. 



Shell internal, rudimentary, uncalcified. No pallial fins in 

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



Section B. Decapoda. 



Arms eight, tvith two clavate "tentacles ;'" suckers pedunculated. 

 Fam. 3. Teuthiclce. 



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

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

 Fam. 4. Belemnitidce. 



Shell internal, composed of a conical chambered portion 

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

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

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



