Fig. 246.—Sepia officinalis L., with mantle cut away to show position of internal shell, × ½. (The ends of the tentacular arms are cut off.)
The Belemnitidae are believed to have been gregarious, and to have lived in shallow water on a muddy bottom. Specimens are sometimes found in which even the ink-sac can be recognised in situ. The relative proportions of rostrum and phragmocone vary greatly in different groups, the rostrum being in some cases two feet long, in others only just enclosing the phragmocone. As a rule the rostrum is the only portion which has been preserved.
Fam. 3. Belosepiidae.—Phragmocone short, slightly curved, chambers small, placed at the posterior end of a sepion, rostrum solid, obtuse.—Eocene (Paris, Bracklesham, etc.).
Fam. 4. Belopteridae.—Sepion not known; phragmocone curved, siphuncle on the ventral margin, rostrum well developed, pointed. Principal genus, Spirulirostra.—Miocene of Turin.
These two families, with their small, curved phragmocone and (in the case of the Belosepiidae) large sepion, are clearly intermediate between the Phragmophora and Sepiophora. Some authorities place them with the latter group.
Fig. 247.—Shell of Spirula Peronii Lam. A, Cutside view; B, showing last chamber and position of siphuncle; C, in section, showing the septa and course of siphuncle; D, shell broken to show the convexity of the inner side of the septa; E, portion of a septal neck.
Fig. 248.—Spirula Peronii Lam.: d, terminal sucker; f, funnel; s1, s2, projecting portions of shell, the internal part of which is dotted in. (From Owen and A. Adams combined.)
B. Sepiophora.—Shell internal, consisting usually of (a) an anterior cancellated portion, (b) a posterior laminated portion, the laminae enclosing air. It terminates in a very rudimentary phragmocone and a rostrum, but there is no siphuncle.