^^oZ I. Grabau — Ordovician Fossils from North China (i) 71 



After the initial rapid expansion, the rate of increase of the tube is diminished, 

 being in some cases more nearly that of an ordinary Orthoceras, but more rapid in others 

 so as to produce a very stout structure. In some cases as in A. richthofeni the rate of 

 expansion diminishes again after a while so as to produce a more cylindrical final portion. 

 In rare cases the form is shghtly curved. The cross-section varies from circular to oval. 



The Siphmde. This is generally large and inflated in the camerte so as to 

 produce a pronounced nummuloidal character or a succession of nummuli. * In size the 

 siphuncle varies from less than one fourth to more than half the diameter of the conch. 

 In the rapidly expanding apical end of the shell the siphuncle quickly reaches a large size 

 and thereafter expands very little if at all, although the diameter of the shell may 

 increase. Thus in the older part of A. richthofeni the siphuncle may occupy more than 

 half the diameter of the shell, while in the expanded portion it does not occupy much 

 more than one third that diameter. In position the siphuncle is centran or excentric 

 even submarginal, though it is not alwaj^s possible in sections to determine with certainty 

 that the centran appearing position is not due to the manner in which the section 

 is made. 



The walls of the siphuncle are thickened by secondary deposits of carbonate of 

 lime. Freeh (Richthofen Vol. V. Plate 2 fig. 1) has illustrated a section of A. crassiven- 

 trum Wahl. which shows the manner of thickening of the siphuncular wall. According to 

 this, the portions opposite the ends of the septa and those in the inflated portions in the 

 camerse ave thickened independently, the former in advance of the latter. As a result of 

 this addition of new material, the central cavity is reduced to a narrow central tube or 

 endosiphuncle, from which lateral annular diverticula extend into the inflated portion of 

 the siphuncle. Frequently the thickening has progressed so far that the whole or nearly 

 the whole of the inflated portions (i. e. the diverticula) become filled solidly, leaving 

 only a central more or less cylindrical tube, the endosiphuncle. This is the case in the 

 majority of specimens of A. tani and A. richthofeni though the specimen figured by Freeh 

 (Richthofen V. Plate 2 fig. 8a, ) still shows the presence of these lateral diverticula. 



In some forms, as in A. richthofeni, the deposition of stereoplasm in the siphuncle 

 is more pronounced in the anterior portion, this resulting in the formation of oblique 

 diverticula from the central tube. This is fully described and illustrated under A. 

 richthofeni (See text fig. 19, p. 79, and Plate IX, fig. 4. ) 



A specimen of A. richthofeni in the collection of the Survey, shows the interior of 

 the siphuncle in a fair state of preservation and unfilled by foreign material, the former 



* See the foot - note on page 76 



