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-caleareous 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 sae 
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 3cm. 
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.) ! % 
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 hooklets. 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 Belemnoidea 
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 Belennites. 
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. : 
“Se ew be eet 7 
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 
Family 1. Belemnitidae. de Blainville. 
. . r . e ‘ 
length, provided with hooklets. Ink-bag present. ‘Trias to Eocene. 
1 Tn addition to the principal literature, cited on pp. 502-505, see the following :—Crick, G. C., On t 
Coceoteuthis and Acanthoteuthis (Geol. Mag. [4], vol. IIT. (1896), p. 439, and LV. (1897), p. 1).— 
Huxley, T. H., and Pelseneer, P., Report on Spirula (Appendix Challenger Rep., Zoology, Part 
LXXXIII.), 1895. : 
