115 



ments of a hollow sphere, the sections of which will be disposed Fn 

 oblique lines, as at Plate VIII. Fig. 4 : the obliquity of the lines 

 increasing with the increase of the distance from the centre. This obli- 

 quity, however, may not always depend on this circumstance ; since, 

 in some species, the septa themselves are disposed in an oblique, 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 occasioned may 

 be seen by comparing the representations, Plate VII. Fig. 14, and 

 Fig. 19, a. The first of these figures, Fig. 14, represents a fossil, which 

 has always been so much regarded as an orthoceratites, that if any one, 

 who had studied these fossils, had been desired to point out one which 

 was most decidedly an orthoraceratite, and not a spirulite, he would 

 have immediately referred to this fossil. 



But the acquisition of the slab of marble from which the fossil repre- 

 sented 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 natural state, 

 in relief; and, on the other side, their internal structure is displayed, 

 in numerous sections, by the cutting of the marble. By an examina- 

 tion of the fossils on the rough part of the marble, as well as by exa- 

 mining some of the sections, it will be seen that they bear not only the 

 exact form of the preceding fossil, Figure 14, but possess also a surface 



