200 
NAUTILOIDEA. 
inequality of development may be produced, by which each segment 
is almost evenly rounded in the lower half, while in the upper its 
outline forms a sigmoid curve, as in fig. 28, u b In no case are the 
Fig. 28. Fig. 29. 
Fig. 28. Lateral view of three siplumcular segments of Hnronia olJiqua, Stokes, 
showing I, the convex outline of the lower half of the segment, and u the 
sigmoid outline of the upper half. Nat. size. 
Fig. 29. Four siphuncular segments of Actinoccras Cloud, Barrande, showing 
their evenly rounded contour. Nat. size. 
elements of the siphuncle evenly rounded throughout the whole of 
their contour, as in Actinoceras (fig. 29), their habit being always to 
increase in diameter towards the top. 
As it often happens in Huronia that the vertical diameter of the 
siphuncular segments exceeds the transverse, it results from this 
that the septa are usually more distant from each other than they 
are in Actinoceras Their very rare preservation is probably due to 
their extreme thinness and fragility, which led Dr. S. P. Woodward ^ 
to surmise that they may have been membranous or horny. If that 
was the case, the great strength and solidity of the siphuncle com- 
pensated for the weakness of the shell and septa. 
1 It is difficult to find an explanation for this abnormal mode of growth, but 
it may be connected in some manner with the oblique position of the siphuncle 
in the shell, by which its development on one side has been retarded, or, again, 
it may have a pathological origin. I have only observed this peculiar malfor- 
mation in one species of Huronia {H ohliqua, infra, p. 205), but it is met with 
also in Biscosorus as already stated (aoite; p. 195). 
2 H. minuens, Barrande, forms an exception to this rule, for in it the trans- 
verse diameter of the siphuncular segments greatly exceeds the vertical. 
^ Manual of the Mollusca, 3rd ed. 1875, p. 191. 
