Vol I. 



Grabau Ordovician Fossils from North China 



(i) 43 



unmistakable constriction, after which the expanding shell of the siphuncle becomes 

 annulated. This suggests that the apical portion of the siphuncle was not enclosed by 

 camera'. The annulations (shown well in specimen fig. 12) are oblique, forming on the 

 side an angle of about 60 with the axis of the siphuncle. They become fainter on the 

 ventral side, meeting in a broadly rounded forward curve or saddle. About five of these 

 annulations occur in the space of 20 mm. 



In transverse section, the siphuncle appears slightly broader than high, the 

 ventral surface being somewhat flattened. 



Interior of siphuncle with endosyphosheaths and crystalline lime-filling between 

 them. Two or possibly more of these older endosheaths are indicated. The endocone 

 formed by the final (last-formed) sheath of the most mature individual seen (fig. 12c), is 

 of subcircular section in the upper part, with the ventral surface slightly flattened. In the 

 lower part, or at least in the earlier sheaths, this ventral surface becomes strongly flattened 

 and the greater part of the endocone lies dorsad of the center (fig. lie). It is continued 

 posteriory in the dorso-ventrally compressed endosiphuncle. The thickness of the last 

 endosheath (the wall of the endocone) is about equal to that of the wall of the siphuncle. 



In the specimen shown in fig. 11, the dorso- ventral diameter is 31 mm., the 

 corresponding diameter of the endocone is 11.5 mm., the space between it and the 

 ventral surface being 11.5 mm., while that between its dorsal surface and the correspond- 

 ing surface of the siphuncle is 8 mm. This point is about 13 mm. from the apex of the 

 endocone. 



A series of sections of another specimen shows the following relationship. (Text 

 figs. la-e). 



a b 



Figs 1 a-e. Succesive cross sections of the siphuncle of Piloeerat plalyventrum ( For distances apart, and measure- 

 ments see table p. 44); la, largest section preserved; le, last section before end of endocone. Natural size. 



