Vol I. Grabau Ordovician Fossils from North China (i) 55 



of structure, which cannot effect the general question of homology. Again the filling 

 with solid lime in the primitive shell of Proterocameroccras (and of the socalled siphuncle 

 of the majority of the Holochoanites), while the outer septal spaces of Ortfioceras are 

 generally empty, is another detail, not relevant to the general question of homology. 

 Indeed there is sometimes the beginning of such an organic deposit about the siphuncle 

 of Orthoceras, while in Stereoplasmoceras and in Actinoceras this is the rule. In these 

 genera too, a secondary septum terminates the deposit of organic lime, this supplementary 

 septum or pseudoseptum being comparable to the sheaths of the "siphuncle" of the 

 Holochoanites (see Plates VI - IX, and discussion on a subsequent page of this memoir). 

 Moreover such deposits are also found in certain Endoceratid* such as Vaginoceras 

 oppletum * where both crystalline lime and " pseudosepta " are formed. This, according 

 to our interpretation, is a new feature, developed in the outer camera, and homceo- 

 morphic with, rather than homologous to the filling of the camera) of Stereoplasmoceras 

 and Actinoceras. It apparently represents a repetition of a structure which had its 

 inception in the formation of the inner shell or "siphuncle " of the Holochoanites, and 

 may perhaps indicate phylogerontism in the group. 



We may here consider briefly the subject of lime deposition by the mollusks, and 

 its bearing upon the problem before us. On the basis of some experiments, and the 

 consideration of others by Murray and Irvine, Steinmann (1889) concluded that the 

 precipitation of calcium carbonate by organisms was a purely chemical process, and was 

 due to the formation of ammonia and carbon dioxide through the processes of decay 

 which are constantly going on in the organism. These substances will precipitate calcium 

 carbonate from the sea water where it is present in the form of calcium sulphate (Ca SO 4 ) 

 and chloride (Ca Cl2). Because of the relatively small amount of lime salts which the 

 animal takes into its body, Steinmann assumed that a part of the lime was directly 

 derived from the surrounding medium. Such precipitation could of course take place 

 only on the edge of the shell if the mantle were free, and its shell-building surface in 

 contact with the sea water. It has however been shown that this is not the case, at least 

 not in those forms, chiefly fresh-water mollusks, which have so far been studied, for there 

 the- periostracum or outer horny covering, bends over the edge of the shell and joins the 

 mantle-border by which it is indeed secreted. Thus lime deposition at the growing edge 

 of the shell goes on entirely under cover of organic structures, and unless it can be shown 

 that by some process of osmosis the sea water finds its way into the spaces between the 

 mantle and the shell, direct precipitation of lime salts seems impossible. Physiologists 



* Ruedemann, loc. cit. 1906 p. 415, fig. 4. 



