594 MOLLUSCA 



SUB-KINGDOM VI 



is situated in the hinder portion of the body and is partially enveloped by the 

 mantle. Among extinct Belemnites the internal shell consists of three parts : 

 a chambered cone (phragmocone), which is prolonged forwards on the dorsal 

 side into a delicate corneo- calcareous proostracum, and is inserted at the 

 posterior end into a finger-like calcareous piece called the guard (sheath or' 

 rostrum), (Fig. 1237, C). 



Some living Cuttle-fishes have an elongated-oval, horny, feather-shaped 

 proostracum or "pen" (Fig. 1255), which is situated dorsally in a closed sac 

 of the mantle. It is sometimes extremely thin, and composed of conchyolin 

 or lime carbonate. The gladius or " cuttle-bone," as the shell is called when 

 calcified in some genera, retains a vestige of chambering at its posterior end, 

 but as a rule exhibits no indication of a phragmocone, properly speaking. 



Many living Dibranchiates are gregarious, and swim in the open sea in 

 hordes ; others creep on the bottom or lead a separate existence along rocky 

 shores. They are extraordinarily active, voracious animals, and prey upon 

 mollusks, crustaceans, and fishes. A few species are esteemed as food by man. 

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

 long, and 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. 



Dibranchiates are divided into three sub-orders, as follows : Belemnoidea, 

 Sepioidea, and Octopoda. 



Sub-Order A. BELEMNOIDEA. (Phragmophora, Fischer.) l 



Shell internal, chambered, and the septa traversed by a siphuncle ; conical or more 

 rarely spiral, and (with the exception of Spirula} terminating posteriorly in a calcareous 

 sheath or guard. Arms ten in number, provided with hooJclets. Trias to Recent. 



Save for the genus Spirula, all forms belonging to this sub-order are extinct. Their 

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

 but there are decided differences both in shell-structure and function. Tetrabranchiates 

 have the shell always external, enclosing the body, but in the present group it is 

 enveloped by the soft parts. Direct connection between the Sepioidea and JBelemnoidea 

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

 phragmocone persists in the former 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 Sepioidea. It is possible to explain the 

 entire internal shell of Spirula as homologous with the phragmocone 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 prosiphon, the same as in Tetrabranchiates. 



Family 1. Belemnitidae. de Blainville. 



Shell composed of a conical camerate phragmocone, 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. 



1 In addition to the principal literature, cited on pp. 502-505, see the following : Crick, O. C., On 

 Coccoteuthis and Acanthoteuthis (Geol. Mag. [4], vol. III. (1896), p. 439, arid IV. (1897), p. 1). 

 Huxley, T. H., and Pelseneer, P., Report on Spirula (Appendix Challenger Rep., Zoology, Part 

 LXXXII1.), 1895. 



