8 EECOKDS 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 Ije 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 Nvhere 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 siphonal 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 infoldings, 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 siphonal membrane are 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 filled 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. 

 t Syst. Sil. Boheme, 11., t. 231. 



