DIBRANCHIATE CEPHALOPODS. 293 



In the genus Ptychoceras the shell is, also, a much elongated 

 cone, which is simply bent upon itself once, the two straight 

 portions of the shell being in contact. The range of this genus 

 is the same as that of Hamites, extending from the Lower 

 Greensand to the Chalk. 



Lastly, in the genus Baculites the shell is simply a straight 

 elongated cone, not bent in any way. Baculites corresponds, 

 therefore, with Orthoceras in the series of the Nauttiidce* The 

 range of Baculites is the same as that of the preceding from 

 the Lower Greensand to the Chalk ; but the genus is most 

 abundant in the Chalk itself. 



CHAPTER XXVI. 

 DIBRANCHIATE CEPHALOPODS. 



THE Dibranchiate Cephalopods or Cuttle-fishes are character- 

 ised as being swimming animals, almost invariably naked, with 

 never more than eight or ten arms, which are always provided 

 with suckers. There are two branchia, which are furnished 

 with branchial hearts ; an ink-sac is always present ; the funnel 

 is a complete tube, and the shell is internal, or, if external, is not 

 chambered. 



The Cuttle-fishes are rapacious and active animals, swim- 

 ming freely by means of the jet of water expelled from the 

 funnel. The arms constitute powerful offensive weapons, 

 being excessively tenacious in their hold, and being sometimes 

 provided with a sharp claw in the centre of each sucker. They 

 are mostly nocturnal or crepuscular animals, and they some- 

 times attain to a great size. They may be divided into two sec- 

 tions, Octopoda and Decapoda, according as they have simply 

 eight arms, or eight arms and two additional " tentacles." 



The parts of a Dibranchiate Cephalopod which may be pre- 

 served in a fossil condition are the mandibles, the ink-sac, the 

 shell (if such be present), and the internal skeleton. The occur- 

 rence of the mandibles and ink-sacs of Dibranchiate Cephalo- 

 pods in a fossil state has been already spoken of (p. 273), and 

 need not be further noticed here. An external shell is pre- 

 sent only in the Argonaut amongst living Cuttle-fishes, and 

 similar structures are of rare occurrence as fossils in some of 

 the youngest portions of the earth's crust. The internal 

 skeleton of the Cuttle-fishes differs very much in its characters 



