CHAMBERED ALVEOLUS. 373 
distance. PI. 44', Fig. 7, b, e, e , e '. This horny 
cup formed the anterior chamber of the Belem- 
nite, and contained the ink bag, (c), and some 
other viscera.* 
Thirdly, a thin conical internal chambered 
shell, called the Alveolus, placed within the cal- 
careous hollow cone above described. (PL 44, 
Fig. 17, a. and PI. 44', Fig. 7, b, b'.) 
This chambered portion of the shell is closely 
allied in form, and in the principles of its con- 
struction, both to the Nautilus and Orthoceratite. 
(See PI. 44, Fig. 17, a, b. and Fig. 4.) It is 
divided by thin transverse plates into a series 
of narrow air-chambers, or areolce, resembling 
a pile of watch-glasses, gradually diminishing 
towards the apex. The transverse plates are 
outwardly concave, inwardly convex ; and are 
perforated by a continuous siphuncle, (PI. 44, 
Fig. 17, b.), placed on the inferior, or ventral 
margin. 
We have already (Ch. XV. Section II.) des- 
cribed the horny pens and ink-bags of the Loligo, 
found in the Lias at Lyme Regis. Similar ink- 
bags have recently been found in connection 
with Belemnites in the same Lias. Some of these 
ink-bags are nearly a foot in length, and show 
* This laminated horny sheath is rarely preserved in connec- 
tion with the fibro-calcareous shelly sheath ; but in the Lias at 
Lyme Regis it is frequently found without the shell. Certain 
portions of it are often highly nacreous, whilst other parts of the 
same sheath retain their horny condition. 
