SUBCLASS II DIBRANCHIATA 679 



In size Dibranchiates are extremely variable ; some forms are only 2 or 3 cm. 

 long, others attain gigantic dimensions. Architeuthis, for example, reaches 

 a total length of 12 metres, the body being 2*5 long, and over 2 metres in 

 circumference. Its arms are thick as a man's leg, and the suckers sometimes 

 as large as ordinary coffee-cups. 



Dibranchiate Cephalopods are divided into three orders, as follows : — 

 Belemnoidea, Sepioidea and Odopoda. 



Order 1. BELEMNOIDEA. (Phragmophora Fischer.)^ 



Shell infernal, chambered, and the septa traversed by a sijdiuncle ; conical or more 

 rarely spiral, and {with the exception of Spirilla) terminating ^posteriorly in a calcareous 

 sheath or guard. Arms ten in number, provided ivith hooklets. Trias to Recent. 



Save for the genus Spirula, all forms belonging to this suborder are extinct. Their 

 camerate shells, perforated by a siphuncle, betoken a kinship with Tetrabranchiates, but 

 there are decided differences both in the structure and function of the shell. Tetra- 

 branchiates have the shell always external, enclosing the body, but in Dibranchiates it 

 is more or less enveloped by soft parts. Genetic connection between the Belemnoidea 

 and Sepioidea is apparent, and although their shells differ in form and structure, yet 

 a rudimentary phragmacone persists in the latter at the posterior end of the skeleton. 

 This rudiment is much more perfectly developed in Belosepia of the Tertiary, which is 

 a connecting link between Belemnoidea and Sej^ioidea. It is possible to explain the 

 entire internal shell of Spirula as homologous with the phragmacone of Belemnites. 

 It begins as a globular or inflated protoconch, which is constricted off from the first 

 camera, and is devoid of a cicatrix. The siphuncle originates as a caecal tube, and is 

 continued apicad as a prosij^hon, the same as in Ammonoids. 



Family 1. Belemnitidae de Blainville. 



Shell composed of a conical camerate phragmacone, continued on the dorsal side as a 

 proostracum, and an elongated solid rostrum or guard. Arms ten in number, of equal 

 length, provided with hooklets. Ink-bag present. Trias to Eocene. 



This family, owing to its great morphological diversity and geological importance, occupies 

 a foremost position among Belemnoidea. The shell may be considered as the prototype of that 

 in all Dibranchiates, since it has all the component parts fully developed, whereas in other 

 groups some of these become atrophied or wanting. 



The shell of Belemnites (Fig. 1314) consists of three fundamental portions : (1) A solid 

 calcareous piece, usually much elongated, and of subcylindrical, conoidal or fusiform shape. 

 This is called the guard (rostrum, osselet, gaine, Scheide), and is excavated at its anterior 



^ In addition to the literature cited under the head of Cephalopoda (v. antea) see the following : 

 Angermann, E., tjber das Genus Acanthoteuthis Minister, etc. Neues Jahrb. Miner., 1902, 

 supplem. vol. xv. — Blamville, H. D. de, Mumoire sur les Belemnites. Paris, 1827. — Crick, G. C, 

 On Acanthoteuthis and Coccoteuthis. Geol. Mag., 1896-97, dec. 4, vols iii., iv. — Idem, Notes on 

 Actinocamax. Ibid., 1904, 1907, vols, i., iv. — Idem, On the Proostracum of a Belenniite from the 

 Upper Lias of Alderton. Proc. Malacol. Soc., London, 1896, vol. ii. pt. 3. — Idem, On the Arms of 

 the Belemnite. Ibid., 1907, vol. vii. , no. 5. — Ide^ni, Buccal Membrane of Acanthoteuthis. Ibid., 

 1898, vol. iii. pt. 1. — Idem, On Belemnocamax boweri from the Lower Chalk. Proc. Geol. Assoc, 

 London, 1910, vol. xxi. — Danford, 0. G., Notes on the Belemnites of the Speeton Clays. Trans. 

 Hull Geol. Soc, 1906, vol. v. — Grossouvre, A. de, Quelques observations sur les Belemnitelles, etc. 

 Bull. Soc. Geol, France, 1899, ser. 3, vol. xxvii. — Huxley T. H.,On the Structure of Belemnitidae, 

 etc. Mem. Geol. Surv. United Kingdom, 1864, Monogr. ii. — PlbilUiJS, J., Monograph of British 

 Belemnitidae. Palaeontogr. Soc. , 1865-70. — Suess, ii\, UeberdieCephalopoden-Sippe Acanthoteuthis. 

 Sitzber. Akad. Wiss. Wien, 1865, vol. Ii. — Voltz, P. L., Observations sur les Belemnites. Paris, 

 1827. — Idem,, Observations sur les Belopeltis ou lames dor.sales des Belemnites. Mem. Soc. d'Hist. 

 Nat. Strasbourg, 1840, vol. iii. — Appeliif, A.. Die Schalen von Sepia, Spirula und Nautilus. 

 Kongl. Sveuska Vetensk. Handl., 1893, vol. xxv. 



