ACTIS'OCEEATID^. 
209 
Sp. Char. Shell straight. Section slightly elliptical. Eate of 
increase about 1 in 8. Septa not very wide apart, being about 
4 lines distant from each other where the shell has a diameter 
of 2 inches. 
In a specimen having the following measurements, viz. length 
3^ inches, least diameter 1 1 inch, and greatest 1|- inch, the diameter 
of the siphuncle (at the necks of the septa) is 5 lines at the smaller 
end of the shell, and 3 lines at the larger end. 
The deposits around the necks (anneaiuv ohstnwteurs), which are 
at first strongly developed, gradually die out towards the body- 
chamber (see fig. 32). 
The surface of the shell appears to have been destitute of orna- 
mentation according to M. Barrande’s observations, but he surmises 
that the shell he examined may have lost its external layer. 
Remarl's. ‘‘ The form of the elements of the siphuncle is very 
remarkable, for it exhibits a considerable diminution in their dia- 
meter, up to a certain height, corresponding to a certain period in 
the life of the animal, and to a height in the shell of about 40 mm. 
They are at first nummuloidal and considerably flattened at the 
smaller end, whilst they appear to be almost cylindrical at the 
larger extremity. This modification is not accidental, nor is it the 
result of disease, for we find it persistent in about ten specimens, 
of which we have the longitudinal sections before us. The fact of 
this decrease in the siphuncle, inversely to the growth of the shell, 
is therefore well established in OrtJioceras docens. As we have just 
stated, at the smaller end of our examples the elements of the 
siphuncle are nummuloidal and flattened. Their breadth is to their 
height as 7 : 2, which indicates a considerable flattening. From this 
maximum of breadth a gradual reduction takes place from below 
upwards in the space of from five to six air-chambers, constituting 
the region of decrease which we have just described. Here the 
siphuncle is reduced to a mean width of about 9 mm. (observed 
in aU the specimens), representing a little less than half the 
maximum width, which amounts to 20 mm. 
“ In consequence of this diminution the siphuncle presents a 
conical appearance in the region under description. Above this the 
elements of the siphuncle stiU retain a sHghtly inflated form and 
resemble slightly flattened spheroids, of which the width scarcely 
exceeds the height. They otherwise preserve about the same di- 
mensions, in ascending towards the larger end of our specimens, 
where probably the limits of the air-chambers are reached. 
P 
