115 
by segments of a hollow sphere, the sections of which will be disposed 
in oblique lines, as at Plate VIII. Fig. 4 : the obliquity of the lines 
increasing with the increase of the distance from the centre. This 
obliquity, however, may not always depend on this circumstance ; 
since, in some species, the septa themselves are disposed in an ob- 
lique, or rather undulating direction. 
When it is considered, that of the recent spirula, very few are found 
with more than three or four concamerations of the straight part of the 
shell attached to the spiral, it is not to be wondered at, that the straight 
is so seldom found connected with the spiral part, in the fossil speci- 
mens. In consequence of this circumstance, some difficulty arises in 
determining which of the straight concamerated fossils are to be con- 
sidered as having been of that form, whilst existing in their complete 
or perfect state, and therefore belonging to the genus Orthocera ; and 
which are to be considered as having originally terminated in a spiral 
form, and which may consequently be considered as the remnant of 
shells of the genus Spirula. An instance of the confusion thus occa- 
sioned may be seen by comparing the representations, Plate VII. 
Fig. 14, and Fig. 19, a. The first of these figures, Fig. 14, repre- 
sents a fossil, which has always been so much regarded as an ortho- 
ceratites, that if any one, who had studied these fossils, had been de- 
sired to point out one which was most decidedly an orthoceratite, and 
not a spirulite, he would have immediately referred to this fossil. 
But the acquisition of the slab of marble from which the fossil re- 
presented Fig. 19, a, was taken, has determined, that such an opinion 
should be adopted with some reserve. It is a slab of light- coloured 
Oeland marble, in which the fossils are seen on one side, in their na- 
tural state, in relief; and, on the other side, their internal structure 
is displayed, in numerous sections, by the cutting of the marble. By 
an examination of the fossils on the rough part of the marble, as well 
as by examining some of the sections, it will be seen that they bear 
not only the exact form of the preceding fossil, Fig. 14, but possess 
