PROFESSOR OWEN ON THE BELEMNITE. 
67 
bottom not rounded, but defined by two parallel lines. The posterior half of the 
guard is slightly compressed laterally, and a very faint trace of a longitudinal impres- 
sion may be discovered on each side, rather nearer the ventral than the dorsal 
surface. One of the specimens of the Belemnites Oivenii, presented to the College of 
Surgeons by Mr. Broderip, measures eleven inches from the apex of the guard to the 
basal partition of the phragmocone: the diameter or breadth of that partition is one 
inch and a half : the length of the guard from its apex to the beginning of the crushed 
alveolus is six inches : the length of the ventral groove is one inch and a half. 
With respect to the guard, I have little to add to the excellent descriptions of 
former authors, besides an account of its microscopic structure*: but this is the 
more requisite, as in some recent and estimable works the spathose structure is still 
regarded as the result of accidental posthumous infiltration. The guard consists of 
numerous, thin, for the most part concentric, layers-}- of minute prismatic trihedral 
fibres^, placed at right angles, or nearly so, to the planes of the layers: the cry- 
stalline fibres are indicated by lines which radiate from the central axis and cross the 
lines of growth : the lines which define the fibres, when magnified 150 diameters, are 
seen in many parts of the section to run in pairs with a minutely and gently undula- 
ting course, resembling the tubes of dentine, but differing in the transparency of the 
intercepted calcareous matter, which is like that in the wider spaces separating the 
pairs of lines. 
These differences in the intervals of the radiating fibres may depend on the different 
parts of the prismatic fibres divided in preparing the sections made parallel to their 
course. 
There is an appearance not uncommon in microscopic sections of the spathose 
guard, which, though due to minute splintering or abrasion of the surface, is too 
characteristic of the texture of the guard to be passed over ; it is produced by a 
number of elongated triangular specks §, defined by their opacity when the section is 
viewed by transmitted light, and by their white or silvery surface when viewed by 
* This was first noticed by Dr. Carpenter in his valuable paper ‘ On the Microscopical Structure of Shells,’ 
in the following words : — “ The solid conical sheath of the Belemnite is a very favourable subject for micro- 
scopic examination ; and of this I have made numerous sections. The greater part of these appear to have almost 
completely lost any indications of organic structure they may have once presented, being almost or completely 
homogeneous in their aspect. In some, however, I have met with a structure, which so closely resembles that 
of the massive Septaria gigantea, that I have no hesitation in the belief, that the shell of the latter is the nearest 
living analogue of the sheath of the Belemnite. I consider the term ‘ fibro-calcareous,’ applied to the latter by 
Dr. Buckland, to be therefore erroneous; the structure being referable to the general type of membranous 
shells. The membrane seems to have been corrugated in a radiating direction, and as the deposition of cal- 
careous matter followed the same, an appearance of radiating fibrous structure is given on fracture, which 
exactly corresponds with that of Septaria. The only difference between the two cases seems to have been, 
that, in the Belemnite the deposit took place from within outwards, whilst in the Septaria it was from without 
inwards.” — Extract from a Paper on the Microscopical Structure of Shells, by William B. Carpenter, M.D. 
Read to the Royal Society, December 22, 1842. 
f PI. VII. fig. 1, (3. + lb. fig. 1, y ; fig. 2, S. 
§ lb. fig. 1, e. 
