THE STRUCTURE OF SOME PRIMITIVE CEPHALOPODS 



BY R. RUEDEMANN 



Plates 6-13 



Professor Whitfield has described [1886,^ p. 319], as O r tho- 

 ceras brainerdi, a cephalopod from the Fort Cassin 

 (Upper Beekmantown) beds of Fort Cassin Vt, which is also 

 very common in beds of like age outcropping along the shore of 

 Lake Champlain at Valcour N. Y. While the originals of the 

 species exhibit but fragments of the phragmocone and lack the 

 living chamber and the apical parts of the conch, there are in 

 the extensive museum collection of specimens secured at Val- 

 cour, not only conchs which supplement the original material 

 but also a great number of siphuncles which exhibit interesting 

 internal structures.^ These and the peculiarities of the apical 

 portion of the conch have led to the investigation, whose results 

 are herewith presented. An extension of the research to the 

 siphuncles of Piloceras explanator Whitfield, another 

 form which is equally common in the Fort Cassin beds at the 

 type locality and at Valcour, has brought to light homologous 

 structures which are also described here. 



1 Parts of siphuncle 



In a siphuncle of the mature conch of Cameroceras^ 

 brainerdi four well defined parts, succeeding each other 

 in apertural direction, can be differentiated. For reasons of 

 plainer demonstration we will consider them here in the reversed 

 order of origin or in apical direction. The first portion of the 

 siphuncle of this species is entirely empty, as in Orthoceras [see 



ISee list of references. 



2 Subsequently these structures were also found in specimens from 

 Fort Cassin itself, which are a part of the State Museum collection. 



3We use here the older term Cameroceras not differentiating between 

 Cameroceras and Endoceras, as Hyatt has done. 



