﻿154 BRITISH FOSSIL CEPHALOPODA. 



appear to undulate, but not much more than 7°. The siphuncle is moderate in size, 

 and would scarcely be called filiform ; in most it is central ; but it is impossible to 

 separate those in which it gradually gets further and further along the greater 

 diameter, till it is only -^ from the sides. These positions are derived from separate 

 septal surfaces, which from their other characters are presumed to belong to the 

 same species. 



These remarkable septal surfaces have long been known, and have been regarded 

 as belonging to 0. bullatum. Their obliquity cannot be seen, and their narrowness 

 would agree with the last named, but they show no signs of striae on the sides ; and 

 in one example of 0. imbricatum they are seen to be the models of the concave 

 side of the septum, which in this instance shows the vascular impressions (fig. 3d). 

 The thickness indicated by these casts is about -|- their longer diameter, and the 

 sides show a peculiar folding of the shell, which is seen also on the sides of complete 

 examples, as, for instance, on that figured by Phillips as 0. marloense. The great 

 abundance of these casts, and their non-association in groups, show that the 

 chambers of the shell fell off during the life of the animal, while the actual substance 

 has been dissolved in the porous mudstone in which they occur, leaving only a 

 model of the inside. The shape is usually elliptic, but sometimes rather quadrate ; 

 from the neighbourhood of the siphuncle, which is most commonly central, but often 

 approaches one side on the longer diameter, there pass out a number of irregular 

 lines like vascular impressions, which bifurcate as they approach the circumference, 

 and finally end in a close fringe of capillaries round the edge. In addition to these 

 there is a band passing from the siphuncle to some point in the side, not always to 

 either end of the diameter, but varying in its position ; this is elevated on the cast, 

 indicating a depression on the shell itself. It seems to have a tendency to draw the 

 siphuncle after it, for the latter is always nearer the side to which this goes. These 

 structures are probably not peculiar to the present species, since the same may be 

 traced on the concave side of the septum of a living Nautilus, but they are seen in 

 this case, owing to the habit of truncation. The impressions are no doubt 

 produced by the veins of the mantle, and the band by the failure of its two sides 

 to meet in wrapping round the siphuncle. 



Relations. — Some confusion has arisen with regard to this species on account of 

 its having been figured with an excentric siphuncle, but originally described as 

 having a central one. From the proved variability of the position of this organ, 

 both these indications are correct ; but in no case is the siphuncle bulbous. Those 

 therefore which were figured by Barrande (pi. 228) could not belong to it. The 

 specimen figured by Sowerby in the ' Silurian System,' though similar to this 

 species in its approximate, oblique, and undulating septa, agrees better in the rate of 

 increase with 0. perversum, which is differentiated from the present not only in this 

 respect, but by the position of its siphuncle and the convexity of the septa. The 



