(i) 6'J Palxontologia Sinica Ser B. 



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



to wbicli wu have referred tbo only known Cambrian cepbalopod ]^olborthella. This 



genus has been defined as a small orthoceracone with conical-shaped septa, i^ierced 



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 



tliat 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 OrtJu ceras 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 tlie chamljered tyi>e 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 secondarilj'' adopted 



device of outer camene. 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 Pal.eo-Ordovician, and again at the end of Neo-Ordovician 



time, when practically the last memliers 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 intcriiret the pliylogeny of the early cephalopods in the following 



manner: 



(to Trias.) 

 }fiiIocli(jnn ite.'< Orthochoun ifc>' 



(Ordovician) (Ordovician) 



: I'rotdcliuanitv^ 



(Candjrian). 



The (.'i/rloch(i(iultc.^, representeil in our rucks by Stcrcdplasinoccras. Actijinceni^ 

 Cyrtactinoccras. and (^'uiiioceras, may have Ijeen derived from primitive Oriliochoanite^, or 

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

 discovery of the genus Sfereujilu-vinuccras, with characters intermediate Ijetween Oiiltocera^ 

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



We may here briefly refer to the remarkable early Ordovician cepbalopod 

 lJijilira(iiiij)rcr(i>! Hyatt. This has the "siphunck"" divided by tabular alternating with 

 the septa of the camerate outer shell. Both the carnene and the chambers of tlie 

 siphuncle are empty. This genus has sometimes been referri'd to as representing a 

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



