SOME NEW AND RARE CEPHALOPODA. 127 



from the number of the gills in the Nautilus\ The Cephalopods with internal cham- 

 bered shells, heretofore classed with the Siphoniferous Cephalopods which constitute the 

 preceding order, I would join with all the other Naked Cephalopods, to form a second 

 order, under the term Dibranchiata' , having reference to the number of gills, viz. two. 

 This number is constant in all the ' Seiches' of Cuvier, and is associated with the pre- 

 sence of two branchial hearts, besides the single systemic heart, and with an ink-bag : 

 there can be little doubt that the same type of structure is exemplified in the Spirula, 

 from what has been determined respecting its external characters \ 



The subdivision of the Tetrabranchiata must necessarily be determined by the modi- 

 fications of the shell and calcareous parts of the beak, since, excepting in one genus, 

 no other parts of the animals now remain for the study of the naturalist. With reference 

 to the higher or Dibranchiate order, as extended by the admission of the Spirula 

 and Belemnites, we may with propriety adopt the character afforded by the number 

 of cephahc arms as indicative of a primary subdivision, and include the Dibranchiates 

 having internal chambered shells, with the Calamaries and Cuttle-fishes in a tribe called 

 Decapoda, or those which have two long peduncles superadded to the eight ordinary 

 arms. The character afforded by the internal chambered shell seems hardly of sufii- 

 cient value to separate the Cephalopods having that part, as a third tribe distinct from 

 the ordinary Decapods ; for the difierence is at least as great between the minute 

 horny style of the Sepiola and the sepium of the Cuttle-fish, as between this latter 

 and the internal calcareous apparatus of the Belemnite. Moreover, Lamarck's figure 

 and the descriptions of the Spirula demonstrate so close a resemblance between its lo- 

 comotive organs and those of the Cuttles and Calamaries, as to afford additional rea- 

 sons for not placing them further apart than as families in the same tribe. 



The tribe Decapoda of the Dibranchiate order of Cephalopods may be subdivided into 

 four families. Of these the Spirulidce, represented by the Spirula ausiralis. Lam., must 

 be regarded as next in the order of affinity to the Tetr abranchiate group. The Belem- 



' Memoir on the Pearly Nautilus, 1832, p. 56. 



- The introduction of these new words for the primary divisions of the Cephalopods into a science already 

 overloaded with Greeli compounds, requires perhaps some apology or explanation. 



The groups which the terms Dibranchiata and Tetrabranchiata respectively indicate are not equivalent to the 

 orders or primary divisions in any previous classification of the Cephalopods, and I could not, therefore, have 

 adopted the ordinal names of my predecessors without the hazard of ambiguity or error. 



The order Dibranchiata, e. g. differs from the Cryptodibranchiata of M. De BlainviUe, and from the Acetabu- 

 lifera of M. D'Orbigny, in the addition of the families Spirulidce and Belemnitidce ; and my Tetrabranchiata dif- 

 fers from the Polythalamucea and the Siphonifera of the same authors, in the absence of those genera in which 

 the chambered and siphonated shells are internal. Under these circumstances, therefore, where a presumed 

 deeper insight into the organization and affinities of a class of animals leads to an actual modification of its 

 subdivisions, new terms for such modified groups seem preferable to the risk of confusion which would arise 

 from applying the same name to two different collections of objects. 



^ The discovery by Dr. Buckland, of the remains of the ink-bag in the extinct Belemnites, justifies the con- 

 clusion from the laws of coexistence, that these Cephalopods also possessed two gUls and two branchial hearts. 

 VOL. II. PART II. 3 



