70 
PROFESSOR OWEN ON THE BELEMN1TE. 
which its frail walls have been unable to resist ; its basal part is usually squeezed 
flat, but sometimes one of the septa has slipped forwards so as to present its surfaces 
to the planes of pressure, and so retain its original form : one of these, which shows 
likewise its natural slight concavity, demonstrates the marginal position of the siphon 
as in other Belemnites, PI. II. figs. 3 and 7, <• 
The septa are composed chiefly of nacre with a thinner layer of white friable cal- 
careous matter on both surfaces, which is seldom preserved. I have found twenty 
septa in an extent of two inches from the base of the phragmocone ; about an equal 
number of septa dividing chambers progressively diminishing in depth, and more 
rapidly in width, are indicated, by detached phragmocones, to have extended to the 
apex of the socket of the guard. The capsule of the phragmocone consists of a thin 
layer of mixed albuminous and opake calcareous matter, lined with nacre, but with 
a yellowish smooth outer surface. In Lord Northampton’s well-preserved specimen, 
it terminates by a well-defined margin a little in advance of the ink-bag, forming 
there the true peristome of the shell, as shown in fig. 9. 
The entire phragmocone, with its capsule, of these Belemnites from the Oxford 
clay, has been found not unfrequently isolated and detached, having slipt out of the 
alveolar cavity of the guard ; such specimens are squeezed flatter than those which 
have remained in and been protected by the guard. The yielding texture of the 
phragmoconic capsule has commonly caused it to fall into longitudinal folds when 
compressed, after having become detached from the alveolus. In two specimens in 
the collection of Mr. Cunnington of Devizes these folds are well-marked, regular, 
and on the same side of the capsule, forming two longitudinal grooves* in one and 
three in the other, the intermediate risings being so well defined as to appear like 
a structure original and natural to the capsule; but their position is different, and 
the degree of the indentation varies, and in other specimens the partial longitudinal 
impressions are barely discernible. If the change of form caused by the compres- 
sion were not borne in mind, these piles of concave plates would seem not to have 
been adapted to the alveolus, and might thus be made occasions for the revival of 
the exploded idea of De Montfort, that the phragmocone of the Belemnite belonged 
to an animal generically distinct from that which had constructed the spathose 
guard. 
Belemnites whose remains have been left in strata percolated by streams contain- 
ing mineral matter in solution, have the chambers of the phragmocone filled with 
crystals consequent upon such calcareous or siliceous infiltration : in the present 
Belemnites from the Oxford clay, no such infiltration has taken place subsequent to 
interment ; we find only the delicate original calcareous framework of the phrag- 
mocone, crushed and squeezed out of its original shape, whilst the solid part of the 
spathose guard, by virtue of its proper primitive fibrous structure, has retained its 
cylindrical figure. This fact, independently of the microscopic evidence of organiza- 
* PI. II. fig. 6, y. 
