CEPHALOPODA. 433 
an initial chamber or protoconch at the apex of the spiral (per- 
haps represented by a cicatrix in the Nautilus); the suture lines 
marking the chambers tend to be lobed. There is often a single or 
paired ** Aptychus,” perhaps of the nature of an operculum. Most 
of the modern forms seem to be more active than their ancestors, and 
their shells have degenerated. But the line of degeneration is still 
debated. In Nautilus, although the animal lives within the shell, the 
mantle fold is for some distance reflected over it; in the other series 
of Cephalopods this process has gone farther, and, where a shell is 
present, it is enclosed within the mantle fold, and is much reduced in 
size. In the extinct Belemnites the internal shell was straight and 
chambered, but almost concealed by secondary deposits of lime, 
secreted by the walls of the shell-sac, and forming the ‘‘ guard” or 
rostrum. The conical chambered shell, with a siphuncle, is known 
as the phragmacone. It is produced anteriorly into a gladius or pro- 
ostracum. In the extinct Spirudérostra the shell was spiral and 
mostly internal; it has a guard. In Spzrz/a the shell can be caught 
sight of in the young animal, but it becomes surrounded by the 
secondary mantle folds that form the mantle-sac.. It is a spiral 
chambered shell, with a ventral siphon. Its relation to the dorsal and 
ventral surface of the animal is the opposite of that of the Mazdédlus. 
The shell is inside the animal; in Maztd/us the animal is inside the 
shell. It seems that Spzru/a is a swift swimmer at great depths ; 
though the empty shells are often cast ashore, the creature itself is rarely 
seen. In Sefza, the narrowed tip of the ‘‘bone” probably represents 
the remains of the phragmacone; the bulk of the ‘‘ bone” probably 
corresponds to the pro-ostracum in the Belemnites. Besides lime 
there is chitin in the ‘‘Sepia-bone.” In Lo/zgo there is no deposit of 
lime, an organic chitinous pen only being left. In Octopus there is no 
trace of shell at all, and no mantle-pocket, save a trace, in the very 
young animal, 
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