(i) 54 Palssontologia Sinica Scr. B 



In Cameroceras on the 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 Vaginoceras except in such forms as Vaginoceras 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 bulb 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 which it remains in contact with the outer 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 Proteroca- 

 meroceras, Cameroceras, Nanno, Piloceras and Chihlioceras, and perhaps in others. Where 

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

 siphuncular wall is the outer shell 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 within it. 



When we consider the length of this preseptal portion in Prolerocameroceras (15 

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

 solid lime matter (stereoplasm), must have been carried on pari 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 be an element of extreme weakness, and therefore not likely to 

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

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

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

 the endosheaths and solid calcareous matter were formed progressively as the tube grew 

 in length, then it appears that these endosiphonal structures are more primitive shell- 

 features than the camera. In other words, for a considerable period of its early history 

 the cephalopod built only a slender shell, which 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 

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

 of these septa, whereupon the endosiphuncle becomes the homologue of the siphuucle of 

 Orthoceras, and the shell of the "siphuncle " of the young Proterocameroceras the homo- 

 logue of the shell of Orthoceras. That the endosheaths, or septa of the primitive Proteroca- 

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



