372 ITS COMPONENT PARTS. 
London, 1826, and that of Sowerby, in his Min. 
Conch, vol. vi. p. 169, et seq. 
A Belemnite was a compound internal shell, 
made up of three essential parts, which are 
rarely found together in perfect preservation. 
First, a fibro-calcareous cone-shaped shell, 
terminating at its larger end in a hollow cone 
(PI. 44, Fig. 17. and PI. 44', Fig. 7, 9, 10, 
11, 12).* 
Secondly, a conical thin horny sheath, or cup, 
commencing from the base of the hollow cone of 
the fibro-calcareous sheath, and enlarging ra- 
pidly as it extends outwards to a considerable 
* This part of the Belemnite is usually called the sheath, or 
guard: it is made up of a pile of cones, placed one within an- 
other, 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 
infiltrations (subsequent to interment), into the intervals between 
the radiating calcareous fibres of which it was originally com- 
posed. The idea that the Belemnite 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, pro- 
duced on burning this part of a Belemnite, arises from the 
remains of horny membranes interposed between each successive 
fibro-calcareous cone. 
An argument in favour of the opinion that Belemnites were 
internal organs, arises from the fact of their surface being often 
covered with vascular impressions, derived from the mantle in 
which it was inclosed. In some species of Belemnites the back 
is granulated, like the back of the internal shell of Sepia offici- 
nalis. 
