(i) 74 Palxontologia Sinica Ser. B 



nummulus, which area was vacant space and has been filled in by the lime mud after 

 burial of the shell. (Plate VII, fig. 6). 



The material Avhich forms the thickening of the septa is crystalhne carbonate of 

 lime, similar in all respects to that which fills the nummuh of the siphuncle. It is readily 

 distinguished from the mud-filhng of the open spaces, which is a uniform dark calcilutyte. 



The thickening of the septa is not uniform. In a specimen of A. coulingi (Plate 

 VIII, fig. 1) it is comparatively slight in the young or apical part of the shell, becomes 

 most marked in the middle portion, and is comparatively slight in the apertural or 

 last-built portion of the shell. This feature is also shown in a specimen of A. tani (Plate 

 VII, fig. G). In this specimen tlie upper surface of the organic deposit is smooth 

 though somewhat undulating, having the appearance of a definite secondary septum. In 

 this species, the thickening increases slightly near the siphuncle, and then thins a^vay 

 very rapidly, generally with a concavity of surface. 



That this lime-deposit is of organic origin, I. e. deposited by the animal which 

 occupied the shell, is beyond question, for only by such an origin can the uniformity of 

 the deposit be explained. That it was formed on successive floors of the living-chamber, 

 i. e. that each deposit was formed Ijofore the next covering septum was built, seems to me 

 also evident, for there is absolutely no indication that the camerte were in subsequent 

 communication with the animal, the small tubuli of the siphuncle notwithstanding. I 

 would interpret the filHng of the camerie as a process strictly analogous to the filling of 

 the "siphuncle" by similar crystalline lime in the HukjcJioartitcs, the "supplementary 

 septum " or "pseudo-septum " which commonly terminates it, being comparable to the 

 endosheaths (as are also the true septa). Thus after tlie formation of each septum, 

 deposition of lime continued upon it for a time, after which, during a resting stage, a 

 pseudoseptum in close contact with the crystalline lime was formed. This a\ as followed 

 by a forward movement of the animal in the shell, and the formation of a new septum, 

 which thus was distant for a certain space (generally less than half the height of the 

 camera) from the pseudoseptum and crystalline deposit. After that the deposition of 

 crystalline lirno recommenced upon the surface of the new septum. 



Distrlhalion. Actinoceras appears abruptly in the Ordovician rocks of North 

 America and north China. A doubtful species {Af viendax Salter) has been described 

 from the Durness limestone of Sutherlandshire (north Scotland), \^'here it occurs in the 

 higher beds (Balnakiel and Groisaphuil groups), a horizon representing essentiallv the 

 Beekmantown or perhaps early Chazy of eastern North America. * The species has also 

 l>een reported from Skye, and doubtfully from Newfoundland and the Mingan Islands. 



* See Grabau, A.W., Bull. Geol. Soc. Amer. Vol. 27, pp. 568-570, 1916, 



