(i) 72 Palseontologia Sinica Ser. E 



filling having been removed by weathering. From this specimen it appears, that the 

 thickening is not uniform all around the periphery of the siphuncle but rather in the 

 form of bead-like enlargements. (Plate VII, fig. 3). This seems to be analogous to the 

 structure of the siphuncle of A. bigsbyi figured by Foord (Cat. Foss. Cephalopoda Brit. 

 Mus. Plate I. pp. 164-165 figs. 20-22) where a series of tubuli run from the endosiphuncle 

 to the outer rim of the intra-cameral expansions, i. e. the nummuli. These tubuli have 

 also been observed in specimens of A. tani from the Chinese rocks, and probably repre- 

 sent a feature usual in this genus. 



According to Foord, the central tube or endosiphuncle is provided with a distinct 

 wall of which the tubuli are diverticulations. Their number has been estimated by Bronn 

 as 16 in A. bigsbyi, but the number of foramina figured in this species by Foord is very 

 much greater. Those so far observed in Chinese specimens are few, probably not more 

 than 16. 



The significance of these tubes and foramina is not clear. Owen (Palaeontology, 

 1860 p. 85) suggested that they may have served for the passage of blood-vessels to the 

 living membrane of the septal chamber, which would imply that these chambers were not 

 merely empty spaces, cut off as in Nautiloids generally. The thickenings of the wall of 

 the siphuncle were regarded by Hyatt "as strictly homologous with the successive 

 sheaths of the endoseptum of Piloceras and Endoceras ' '. I would however interpret the 

 endocones of the Holochoanites as the crowded septa of an inner shell, comparable to an 

 orthoceracone, which would make them entirely distinct from the thickening of the 

 siphuncle of Actinoceras. 



Suggestions have repeatedly been made regarding the significance of these 

 siphuncular thickenings. Freeh suggests that the thickening represents an attempt to 

 render mechanically weak cylindrical structures more resistant against wave and current 

 attack, the siphuncle thus being transformed into a supporting structure, or into a species 

 of back-bone. That the solidified siphuncle became such a supporting structure, and that 

 because of it the genus Actinoceras was a long-lived one, extending from the Ordovician 

 to the Carboniferous, may be conceded, though it is by no means certain that Actinoceras 

 as now understood is monophyletic. In other words it is not improbable that the 

 Actinoceras type of siphuncle was independently developed in more than one phyletic 

 series, representing thus parallelism in development, rather than genetic relationship. 

 The origin of the structure however must be sought in the purely mechanical processes of 

 lime separation as the result of the decay of the cells of the older part of the siphuncle, 

 the gradual contraction of which was a concomitant phenomenon of the functional 

 detereoration of that part of the animal's anatomy. I would regard this excessive lime 



