CEPHALOPODA. 
279 
extending parallel with it. It is obscured, towards the central portions of 
the tube, by an organic deposit on each side of the median line, having a 
symmetrical form, and presenting the appearance of a sculpturing, or scarring 
of the ventral walls, and consisting of a palmate arrangement of ridges and 
nodes of the organic deposit. This ornamentation on the internal mould is 
finally obliterated near the apex by the predominance of the organic deposit 
in the air-chambers. The organic deposit is formed over the walls of the 
air-chambers, and over the surface of the septa, consisting of a vesicular 
aggregation of calcareous matter, similar in some respects to 0. luxum of the 
Schoharie grit; and, as in that species, it increases in amount and com¬ 
plexity toward the apex. 
Siphuncle excentric, near the ventral side, straight, cylindrical, slightly 
constricted in its passage through the septa. The distance from the dorsal 
and ventral sides are as ten to eighteen, and its diameter is four mm. where 
the tube has a diameter of thirty mm. 
Test strong, having a thickness of from one to one and a half mm. Sur¬ 
face marked by fine, regular, slightly-curving striae of growth, eleven to 
seventeen in the space of four mm., crossed by indistinct, irregular, longi¬ 
tudinal striae. Towards the apex the longitudinal striae are sharper and 
stronger, becoming more prominent and regular than the concentric lines, 
which are here more crowded than in the anterior portions of the tube. A 
broad, gentle sinus in the direction of the striae, over the siphuncle, indicates 
the position of the ventral side. 
The internal mould is obscurely, longitudinally striate, and the chamber 
walls essentially smooth and flat, with the excepdon of the characteristic 
ornamentation of the ventral walls. Septal sutures but slightly indented. 
One of the largest individuals has a length of forty centimetres, with a 
diameter at the aperture of nearly fifty mm., embracing the chamber of habi¬ 
tation and about sixty air-chambers. 
This species is quite variable, both in proportions and in general appearance, 
as may be seen by consulting the illustrations of the species. The variation 
in the character of the surface-markings is quite conspicuous on different 
