8 
RECORDS OF THE AUSTRALIAN MUSEUM. 
River, less the oldest and youngest chambers of the shell, and 
unfortunately it has been crushed, more particularly in the 
upper portion of the specimen. The length is six inches, and 
there are within this space nineteen or twenty chambers, the 
upper with a breadth of one-quarter of an inch, and the lower a 
trifle less. At both ends the large beaded siphuncle is visible, 
above in the round, below in partial cross-section. 
The siphuncle is nearly marginal in position, or in a perfect 
specimen would probably be sub-marginal. At the younger end it 
stands out from the crushed and partially denuded shell exhibiting 
portions of three of the “beads,” or segments, so characteristic of 
the genus. The diameter in its present condition is nine-sixteenths 
of an inch, but at the older or lower end of the shell it is only 
three-sixteenths. The siphuncular segments to the naked eye 
are grooved and ridged, and where not abraided, the ridges are 
very slightly convex. An examination of the partially and 
naturally sectioned siphuncle at the older end of the shell, as 
well as in a cut section, reveals the fact that these grooves are 
the infolding of the siphomil membrane, as described by Mr. 
A. H. Foord* who says: “The calcified lining membrane of the 
siphuncle is thrown into a series of folds, which impart to it a 
puckered appearance, which is very characteristic.” The same 
Author also observes that the shelly covering of the siphuncular 
segments, or “beads” composed of several layers, is very rarely 
preserved, but at the oldest end of the present specimen it is 
distinctly visible. Some good figures of the infolding of the 
membrane are extant, and foremost amongst these may be men- 
tioned Actinoceras Bigsbyi , Stokes, as represented by Barrande.f 
In some of the in foldings, the membrane seems to expand into 
vertical sac-like cavities protruding inwards. When subjected to 
microscopic examination, in a thin section, the inflected portions 
of the si phonal membrane arc seen to be comparatively thick, 
each one increasing slightly in width as it proceeds inwards, 
becoming somewhat truncheon-shaped, leaving in the centre a 
narrow free space tilled with impalpable matrix. They are 
variable in length, some long, some short, but never approach- 
ing the centre of the siphuncle. At the point through which 
the section is taken there are seventeen of these inward pro- 
longations, but they do not appear to be developed with equal 
regularity as to distance apart around the rather oval siphuncle. 
Furthermore, these prolongations appear to be open to variation 
in shape, for along one side are two assuming a decidely 
pyriform outline, and a third that seems to show signs of 
bifurcation at its inward end, although too much stress must 
not be laid on this point. There is no trace of the endosiphon, 
* Cat. Foss. Ceph. Brit. Mus„ 1888, Pt. i.., p. 166. 
f Syst. Sil. Boheme, ii., t. 231. 
