378 
ANALOGIES. 
most important parts to be nearly complete ;* 
and the same analogies might be traced through 
the other genera of chambered shells. f 
* The air chambers and siphuncle are, in both these families, 
essentially the same. 
In Belemnites, the anterior extremity of the fibro-calcareous 
shell, which forms a hollow straight cone, surrounding the 
transverse plates of the chambered alveolus, represents the hol- 
low coiled-up cone containing all the transverse plates, which 
make up the alveolus of the Nautilus. 
The anterior horny cup, or outer chamber of the Belemnite, 
surrounding the ink bag, and other viscera, represents the large 
anterior shelly chamber which contains the body of the Nautilus. 
The posterior portion of the Belemnite, which is elongated 
backwards into a fibrous pointed shaft, is a modification of the 
apex of the straight cone of this shell, to which there seems to 
be no equivalent in the apex of the coiled-up cone of Nautilus. 
The cause of this peculiar addition to the ordinary parts of 
shells, seems to rest in the peculiar uses of the shaft of the Be- 
lemnite, as an internal shell, acting like the internal shell of the 
Sepia Officinalis, to support the soft parts of the animals, within 
the bodies of which they were respectively enclosed. The fibrous 
structure of this shaft is such as is common to many shells, and 
is most obvious in the Pinnae. 
f Comparing the Belemnite, or internal shell of Belemno- 
sepia with the Sepiostaire, {Blainville), or internal shell of the 
Sepia Officinalis, we have the following analogies. In the 
Sepiostaire, (PI. 44', Fig. 2, a. e. and Figs. 4, 4', 5), the small 
conical apex (a) represents the apex of the long calcareous pos- 
terior sheath of the Belemnite, (Fig. 7, a.), and the calcareous 
plates, alternating with horny plates, which form the shield and 
shallow cup of the Sepiostaire, (PI. 44', Fig. 2, e. and Fig. 5. e.)> 
represent the hollow fibro-calcareous cone or cup of the BelerO' 
nite, surrounding its alveolus. 
The margin of the horny plates, interposed between the cal' 
careous plates of the shield and cup of the Sepiostaire, (PI. 44 > 
Fig. 4, e, e, e', e'.), represents the horny marginal cavity of d’® 
