(i) 54 I'ahvoiitoloijia Sinica Ser. B 



In Cameroceras on thu other hand the nepionic bulb or swollen end of the 

 siphuncle, is largely or wholly surrounded by camera', and this is also the case in 

 Kndoceras, and generally in J'uginoceras except in such forms as Vaginocems belemnitiforme 

 Holm. These two genera differ from Cameroceras and Proterocameroceras in the absence 

 of the siphuncular wall or shell, (the endosipholining of authors). 



Another genus in which this preseptal cone or nepionic bullj exists before the 

 camerate portion begins, is Nanno Clarke, also of Middle and early Upper Ordovician 

 (Chazy and Black River) age. In this genus the siphuncle is strongly contracted at the 

 beginning of the camerate portion, after ^vhich it remains in contact with the (juter shell 

 of the camerate portion, on the ventral side. 



The presence of the siphonal wall or shell (endosipholining) in the more 

 primitive genera is of marked significance. This wall is known to occur in Pruteroca- 

 meroccras, Cameroceras, Naiino, PUoceras and ChihUoceras, and perhaps in others. A\'here 

 the shell Ijegins with a non-camerate apical portion, i. e. with only the siphuncle, this 

 siphuncular wall is the outer sliell of the cephalopod hard structure. In other words the 

 young cephalopod began shell-building with the "siphuncle" which consisted of the 

 siphuncular shell-wall and the filling M'ithin it. 



A\'hen we consider the length of this preseptal portion in Pruterocamcroceras (7.") 

 mm. in P. hrainerdi) it is evident, that the filling of the interior Ijy endosheaths and 

 solid lime matter (stereoplasm), must have been carried on [lari passu, with the building 

 of this shell, after the formation of a short initial hollow conical tube. For not only would 

 such a long hollow tube lie an element of extreme weakness, and therefore not likely to 

 be preserved, lait also, it is diflicult to conceive that the cephalopod grew into such a long 

 rod-like body, before it began the building of eamene, and that this budv soon there- 

 after Ijegan to shrink into the slender thread which occupied the endosiphuncle. But if 

 the endosheaths and solid calcareous matt<jr were formed progressively as the tube grew 

 in length, then it aj^pcars that these endusiphonal structures are more primitive shell- 

 features than the camerte. In other words, for a considerable period of its carlv history 

 the cephalopod built only a slender shell, wliich it progressively filled with calcareous 

 matter, marked at certain periods by resting stages, when the conical endosheaths were 

 built. If that is the case, the endosheaths have the same significance, in these primitive 

 shells, as the septa have in a shell of Orthoceras, and must be considered the homologues 

 of these septa, whereupon the endosiphuncle liecomes the homologue of the siphuncle of 

 Or//toceras, and the shell of the "siphuncle" of the young Proierucnmeruceras the homo- 

 logue of the shell of Orthoceras. That the endosheatlis, or septa of the primitive Prokroca- 

 ■meroceras are deeply conical, while those of Orthoceras are saucer-shaped, is only a detail 



