284 



MINERALS AND GEOLOGY 



Cytolites, Maclurea, &c., formerly regarded as belonging to this cla 

 are now placed under the holostomatous gasteropoda. 



CLASS VI. CEPHALOPODA. 



This class is represented by highly-organized mollusca, furnished 

 with a distinct head, large eyes, and a central mouth (with horny, 

 beak-like jaws) surrounded by a series of tentacles, or so-called 

 " arms/' which serve as organs of prehension and locomotion. The 

 Nautilus, Argonaut, Octopus, Sepia or Cuttle-Fish, and Loligo or 

 Squid, are its principal living representatives. These are commonly 

 classed under two leading sections or orders, Tetrabranchiata and 

 Dibranchiata, in accordance with thj number of their branchiae. 

 The Nautilus is the only living representative of the Tetrabran- 

 chiate Cephalopods, but many extinct genera in which the character 

 of the shell is more or less akin to that of the Nautilus, are usually 

 regarded as belonging to the same order. Of late years, however, 

 doubts have arisen on this point with respect to some of these extinct 

 forms, which are now supposed to have been more nearly related (as 

 regards the animal, apart from the shell) to living Dibranchiate 

 types. They are chiefly represented by the extinct ammonites and 

 related genera, Hence, the recent adoption of three sections or 

 orders in the classification of the Cephalopoda namely : (1) Tetra- 

 branckiata ; (2) Ammonoidia, and (3) Dibranchiata. The Tetra- 

 branchiata are essentially ancient types in a state of decadence, the 

 Nautilus (with greatly diminished species) being the only living 

 representative. The Ammonoidea are extinct, typically Mesozoic, 

 forms. The Dibranchiata, on the other hand, although including 

 some extinct genera (as the Belemnite of Mesozoic age,) are essen- 

 tially recent types. These relations are shown in the following 

 diagram : 



Existing Period. 

 Cainozoic Periods. 



Mesozoie Periods. 

 Palaeozoic Periods. 



FIG. 227. 



Diagram, shewing geological relations of the 'retrabranchiate, Ammonitoidal, and 

 Dibranchiate Cephalopods, respectively. 



