CHAMBERED ALVEOLUS. 281 



Secondly, a conical thin horny sheath, or cup, com- 

 mencing from the base of the hollow cone of the fibro-cal- 

 careous sheath, and enlarging rapidly as it extends outwards 

 to a considerable distance. PI. 44', Fig. 7, b, e, e', e". This 

 horny cup formed the anterior chamber of the Belemnite, 

 and contained the ink-bag, (c,) and some other viscera.* 



Thirdly, a thin conical internal chambered shell, called 

 the Alveolus, placed within the calcareous hollow cone 

 above described. (PI. 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 construction, 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 areolcB, resembling a pile of 

 watch-glasses, gradually diminishing towards the apex. 



is made up of a pile of cones, placed one within another, having a common 

 axis, and the largest enclosing all the rest. (See PI. 44, Fig. 17.) These 

 cones are composed of crystalline carbonate of lime, disposed in fibres that 

 radiate from an eccentric axis to the circumference of the Belemnite. The 

 crystalline condition of this shell seems to result from calcareous infiltra- 

 tions (subsequent to interment,) into the intervals between the radiating cal- 

 careous fibres of which it was originally composed. The idea that the Be- 

 lemnite was a heavy solid stony body, whilst it formed part of a living and 

 floating sepia, would be contrary to all analogies afforded by the internal 

 organs of living Cephalopods. The odour, resembling burnt horn, pror 

 dueed on burning this part of a Belemnite, arises from the remains of 

 horny membranes interposed between each successive fibro-ealcareous 

 cone. 



An argument in favour of the opinion that Bclemnites were internal 

 organs, arises from the fact of their surface being often covered with vascu- 

 lar impressions, derived from the mantle in vvhich it was enclosed. In some 

 species of Belemnitcs tiie back is granulated, like the back of the internal 

 shell of Sepia officinalis. 



* This laminated horny sheath is rarely preserved in connexion 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 !;etain their horny 

 condition, 



24* 



