68 
PROFESSOR OWEN ON THE BELEMN1TE. 
reflected light : the long axis of these triangles is always parallel to the fibres, and 
transverse to the layers ; but on one side of the section the apices are turned towards 
the centre, on the other side towards the circumference of the guard ; so that a slight 
change of focus reverses the position of the triangular specks. 
The lines produced by the concentric layers of growth are seldom marked with 
equal distinctness ; the strongest ones are usually in groups of three, five, or eight, 
with fainter lines in the clearer interspaces : this appears due to the varying thick- 
ness of the animal membrane at the contiguous surfaces of the layers of the prismatic 
fibres, which membrane was either formed by, or afforded attachment to the extre- 
mities of the delicate membranous cells which served as moulds for the calcareous 
matter of the fibres. The impress of these extremities has produced on many of the 
lines of growth a minutely undulating or crenate course. The layers of growth vary 
slightly in thickness ; many are brought into view by applying a magnifying power 
of 150 diameters to a transparent section of the guard which otherwise would escape 
notice ; those only being visible to the naked eye that are separated by the thicker 
lines ; they are thus seen to be much more numerous than the septa of the phragmo- 
cone. In a transverse section through the middle of the alveolus, eighty layers could 
be counted in a thickness of a line, and more than three hundred in the solid part of 
a guard whose semidiameter was four lines. 
A longitudinal section of the guard through its centre, and a transverse one, both 
demonstrate the longitudinal course of the radiating fibres, the linear indications of 
which might be interpreted as the folds of a plicated membrane : a longitudinal sec- 
tion taken near the circumference of the guard cuts across the radiating fibres, the 
extremities of which are thus seen in transverse section ; their distinct and indepen- 
dent character and trihedral form are then clearly demonstrated, as in PI. VII. fig. 2 : 
they vary a little in size, the average diameter being Yoircdh of an inch. 
Through a great part of the thickness of the guard next the outer surface, the thin 
fibrous conical layers are concentric, progressively increasing in length as they 
approach that surface, and thus forming the alveolus or cavity for the phragmocone. 
The innermost or first-formed layers are not parallel with the outer ones, but recede 
from them at their anterior end, contracting as at the opposite end, but in a less 
degree, so as to form a slender cylindrical stem, sometimes slightly dilated at the end 
which is in contact with the apex of the capsule of the phragmocone. The interspaces 
thus left between the earlier and the later layers of the guard are occupied by more 
abundant animal membrane, mixed with coarser opake calcareous particles, like 
those which harden and render brittle the corneo-nacreous capsule of the chambers ; 
a filamentary tract of the same kind of matter is continued from the apex of the 
above capsule down the centre of the guard to its posterior apex. 
Mr. Pratt has obtained from the Oxford clay at Christian-Malford the guard of 
a young Belemnite, three lines long*, fusiform, slightly contracted posteriorly; 
* PL II. fig. 4. 
