Vol I. Grahau — Ordovician Fossils from North China (i) 37 



Shell oval in section, the largest fragment known having a derso-ventral diameter 

 of 23 mm., while the transverse diameter is something over 30 mm. At the same point 

 the siphuncle has a diameter of 12 and 15 mm. respectively, thus being at tliis stage about 

 one half the size of the shell. The ventral side is dis'iii:ctly flattened, this being shown 

 both in the shell and in the siphuncle, Avhich lies in close juxtaposition to the flat ventral 

 side of the shell. The thickness of the shell- wall at this point is half a millimetre. The 

 camerte average 3 mm. apart at this stage, and their concavity, as nearly as can be 

 ascertained, is equal to about twice tlicir distance apart at the center, or a little more. 



Siphuncle distinctly flattened on tlie x'L'ntral side where it is in contact Avith the 

 shell, the flattened part in the apical portion of the largest specimen being 9 mm. It 

 tapers at the rate of about 1 mm. in 16. There is a well-defined and distinct wall, which 

 encloses the siphuncle {endosliihollning of Ruedemann). This siphuncular wall or inner 

 shell is obliquely annulated by tiio edges of the siphonal necks, which are shghtly but 

 distinctly constricted just before reaching the next lower septum. The suture forms a 

 distinct ventral saddle on the flattened surface of the shell. The anterior empty portion 

 of the siphuncle (the endosipJiocyUnder of Ruedemann, i. e. the inner living-chamber) is 

 broken away to tlic edge of the last-formed endosipbosheatli (inner conical septum). The 

 depth and apical angle of the endosiphocone, delimited by this last sheath, cannot be 

 ascertained, but the former is at least 20 or 25 ram. \\'hich would make the latter about 

 25 . The interior of the siphuncle below this cone is fllled by crystalline calcite which 

 has a distinct radial structure as in belemnites. In its center or nearly so, lies the 

 subtriangular cndusipliocoleon, flattened on the ventral side to correspond to the flat veiitral 

 face of the siphuncle. Its ventral diameter at the lower end of the specimen is about 

 2.5 mm. 



Another specimen (Plate IV fig 2a-c) shows an earlier portion of the shell, 

 apparently of a different individual. Tlie shell is strongly oval, the masimum transverse 

 diameter being 18 mm. while the dorso-ventral diameter is only 11 mm., though the 

 shell appears to be somewhat crushed dorsally. The corresponding diameters of the 

 sipliuncle are 8 and 7 mm. respectively. 



The septa are a little over 1 mm. apart. The siphuncle is in close juxtaposition 

 to the fiat surface of the shell, which is about 6 mm. wide. The sides of the siphuncle are 

 obliquely annulated by the septal necks, which form an angle of TO*' with the ventral 

 surface of the siphuncle, this being essentially the angle formed by them in the larger 

 specimen. The endosiphocoleon lies somewhat dorsad of the center. Rate of tapering of 

 siphuncle 1 mm. in 26. 



