Vol I. Grabau Ordorician Fossils from North China (i) 67 



one third the diameter of the shell towards the center. At first they are more or less 

 parallel to the septum, then slope more or less abruptly to the septal surface near the 

 center where they either join the septum, becoming confluent with it ; or continue as an 

 independent layer in contact with the septum at the center. At the shell margin these 

 pseudosepta again join the main septum. The pseudosepta on the under side of the true 

 septa are more irregular. As seen in section, some are annular, joining the main septum 

 in the center as do the pseudosepta above. In other cases the pseudosepta diverge from 

 the main septum some distance in from the shell-margin, and continue across the center 

 to within a similar distance of the opposite shell-margin. These pseudosepta thus have a 

 greater curvature than that of the true septa. When the pseudoseptum is confined to the 

 marginal portion of the section, it is in close contact, for a space, with the pseudo- 

 septum which joins the next preceding septum on the upper side. Again, the pseudo- 

 septum on the under side may become irregular, as it approaches the center, being 

 abruptly bent down, before it bends up again to join the under side of the septum above 

 it. The space between the pseudosepta and the septum to which they belong both above 

 and below this true septum, is filled solidly with stereoplasm in the form of crystalline 

 calcium carbonate (probably aragonite, at least originally). Thus in general the septa 

 appear thickened on both upper and lower marginal portions by nearly equal amounts of 

 stereoplasm, while the center is free from such thickening, the carnerce being filled only 

 by the lime-mud in which the shell was buried. In the older parts, where the lower 

 pseudosepta seem to extend across the center (possibly due to the position of the section) 

 nearly the whole of the camera appears filled with the stereoplasm. 



The siphuncle is excentric, situated about half-way between the center and the 

 margin of the slightly flattened side, or a little nearer to the latter. Around it the 

 camera are often free from stereoplasm for some -considerable space. The siphuncle is 

 nummuloidal, expanding to 7 mm. in the center, where the septal distance is 4 mm. but 

 it does not appear to be regular. At the septa it contracts to about 2.5 mm. There are 

 either no stereoplasmic deposits, or, when present, they are irregular, and have the 

 nature of a norrower tube within the more expanded outer nurnmulus. Characters of 

 exterior of shell and of living chamber unknown. 



A section of a specimen of Stereoplasmoceras from the Machiakou limestone of 

 WGn-nan, Shantung (Plate IX fig. 11), appears to belong to this species, representing 

 the earlier portion of the conch. The shell tapers at the rate of 1 in 6. The siphuncle, 

 though appearing centran in the section, is only about 4 mm. from the ventral margin at 

 the lower end of the specimen, and 5.5 mm. at the upper end. The diameter of the 

 nummuli is 7 mm. where the shell section is 28 mm. wide, in the upper end of the 



