Vol /, Orabau — Ordovician Fossils jrom North China (i) 81 



the shorter axis of the section, and distant from the nearest shell-margin a variable 

 amount, this ranging from about half, to a little more than its full diameter. From the 

 opposite shell-margin it is separated between two to three times this amount. In a 

 specimen from Tangshan (Plate VII, fig. 5, cat. no. 26) the diameter of the siphuncle is 

 5 mm. where the shell is 22.3 mm. in diameter, while in another specimen from 

 Shantung, the siphuncle has a diameter of 4.5, the shell diameter being 17.5 mm. (cat. 

 no. G6). In the specimen illustrated in figure 6 Plate VII, (cat. no. 1), where the 

 siphuncle is apparently centran, because of the position of the section, it retains a very 

 uniform diameter of 6 mm. while that of the shell ranges, at the point of the section, 

 from 16 to 20 mm. As this is a shell with excentric siphuncle,* the section does of 

 course not represent the maximum diameter nor the true rate of tapering. 



In a specimen from Chihli (Plate VII, figs. 4a, b, cat. no. 69) which shows the 

 greatest rate of expansion observed (1 in 5), the siphuncle appears centran in longitudinal 

 section, but the transverse section shows it to be about one half its diameter from the 

 margin. The siphuncle is 8 mm. in diameter where the shell in the same section is 24 

 ram. The endosiphuncle (endosiphotube) is about 1.5 mm. in diameter or a little more, 

 and appears as a regular cylindrical tube. The septa average 2.6 mm. apart. They are 

 sometimes double, and the cameras are largely filled with stereoplasm. 



The nummuli, or expansions of the siphuncle in the camerte, are generally 

 regular and symmetrical, though now and then one is shorter, or of smaller diameter. 

 When well exposed, the peripheral mural pores may be seen, ranged around the ambitus 

 of the nummulus. In a specimen from Tangshan (Plate VII, fig. 7c), they are small, 

 and each is situated at the summit of a low pustule, the pustules being separated by a 

 space equal to about twice their diameter. I estimate their number on a single nummu- 

 lus to be about 24. 



The septa are strongly thickened by the addition, on the upper side, of stere- 

 oplasm, which has a crystaUine structure and is terminated by a smooth - surfaced 

 supplementary septum or pseudoseptum in each camera. The thickening proceeds to 

 within a short distance of the siphuncle, when it dies away abruptly, the pseudoseptum 

 sloping steeply and generally concavely to the septum, which latter joins the siphuncle in 

 the constriction between the nummuli. Thus a sort of saucer-hke depression is formed 

 around each nummulus, which appears to lie in it like the pudding in a dish. 



The thickening, by stereoplasm, is not uniform ini [successive camera3, nor within 

 the same camera. As Avill be seen from the natural section of the specimen illustrated in 



* This can not be positively seen in the specimen in question, but as it lias all the other characters of this 

 species, which from other specimens is known to have an excentric siphuncle, the above inference may be safely made. 



