Vol J. Grabau Ordovician Fossils from North China (i) 73 



separation rather as evidence of senescence, and consider the Actinoceran type a phylo- 

 gerontic phase of orthoceraconic development. 



The apertural end of the siphuncle. A specimen of A. richthofeni from Huo-Luh, 

 Chih li province, presented to the Survey collection by Miss Clarke, shows a section of 

 the apertural end of the siphuncle (Plate VII, fig. 2). This has a, maximum diameter 

 of 12 mm. and shows a conical depression about 25 mm. in depth and 8 mm. across at 

 the upper end. The sides of the apertural cone are formed by the obstruction rings or 

 rosettes of. stereoplasm deposited about the septal necks, this deposit being slight in 

 the upper part and increasing in thickness downward. This gives the inner surface of the 

 cone an undulating appearance, contracting at the septal ne.cks, and expanding between the 

 septa, in conformity with the expansion of the siphuncle. The continuation of the cone 

 in the endosiphuncle is not shown in the section, but undoubtedly existed. The 

 funnel-shaped apertural end of the endosiphuncle resembles that of the Silurian genus 

 l)i*cosorus Hall, but there is no lining membrane or sheath as in that genus. It 

 merely represents the still unfilled portion of the siphuncle and shows that the filling by 

 "obstruction rings " progressed regularly from behind forward. There are indications in 

 the specimen thai the camera? of the shell continued beyond the upper end of this cone 

 thus suggesting that the upper end of the siphuncle consisted of a series of hollow 

 nummuli. If this was the case, the upper portion corresponds in character to the genus 

 Stereoplasmoceras and as it represents a more primitive stage in development through 

 which the shell passed as a whole (the filling being subsequent to the formation of the 

 nummuli) the suggestion lies near that Actinoceras is a derivative from Stereoplasmoceras, 

 which in turn is derived from Loxoceras, and that from Orthoceras. 



The septal thickening. In practically all of the specimens of Actinoceras from 

 the Machiakou limestone, a striking thickening of the septa by stereoplasm or 

 organic deposits of carbonate of lime has taken place, so that the camera? are more or less 

 completely rilled by this calcareous deposit. Complete filling is rare, but has been 

 observed in some cases, while in others the thickening has proceeded only far enough to 

 fill about one half of the camenL The thickening is most generally produced by 

 addition of lime to the upper surface of the septum, but in other cases it appears to be 

 added to the under side as well. It is however possible that this appearance is deceptive, 

 and due to the irregularity of the septum, which bends forward before reaching the 

 inter-nummuloid contractions of the siphuncle (see A. coulingi, Plate VIII, fig. 1 cat. no. 

 4). The deposit is very often thickest near the siphuncle, close to which it frequently 

 thins away abruptly, leaving a subtriangular area next to the outer margin of the 



