(i) 62 Palseontologia Sinica Ser B. 



ancestral stock is represented by the sub-order, Protochoanites Grabau and Shimer, 

 to which we have referred the only known Cambrian cephalopod Volborthella. This 

 genus has been defined as a small orthoceracone with conical-shaped septa, pierced 

 by a simple hole in the apex, though the presence of such a siphuncular hole has 

 been questioned. Whether or not it occurs is of secondary importance as it appears 

 that in some of the Holochoanites the " endo-siphuncle ' is not present in the 

 earliest part. But whether the known Cambrian protochoanite is the direct ancestor 

 of both Orthccems and the Holochoanites is of little moment. Certain it seems that shells 

 so distinctive must represent divergent evolutional lines, one the orthochoanite branch 

 becoming successful by the assumption of the chambered type of shell, the other, the 

 holochoanite type disappearing with the close of the Ordovician, because its heavily 

 weighted shell made active migration impossible in spite of the secondarily adopted 

 device of outer cameree. The extinction of the various genera of this group is in general 

 coincident with great changes in physical geography, notably the extensive withdrawal 

 of the seas at the end of the Palfeo-Ordovician, and again at the end of Neo-Ordovician 

 time, when practically the last members of the group perished. Nor is it an unwarranted 

 assumption that the excessive development of the sedentary habit may have affected the 

 reproductive powers of these animals in one way or another, so that extinction may be in 

 part due to this cause. 



We would then interpret the phylogeny of the early cephalopods in the following 



manner: 



(to Trias.) 



The Cyrtochoanites, represented in our rocks by IStereoplasmoceras, Actinoceras. 

 Cyrtactinoceras, and Gonioceras, may have been derived from primitive OrthocJioanites, or 

 they may represent an independent line of evolution from the ancestral stock. The 

 discovery of the genus Stereoplasmoccras, with characters intermediate between Orthoceras 

 and Actinoceras, makes the former view the more likely one. 



We may here briefly refer to the remarkable early Ordovician cephalopod 

 Diphragmoceras Hyatt. This has the "siphuncler" divided by tabular alternating with 

 the septa of the camerate outer shell. Both the cameras and the chambers of the 

 siphuncle are empty. This genus has sometimes been referred to as representing a 

 transition from the Holochoanites (Endoceran type) to the Orthochoanitcs (Orthoceran 



