8 3 8 



DIVISIONS OF THE CEPHALOPODA. 



The type-genus of this family is Endoceras, in which the shell is 

 straight and resembles that of Orthoceras in form. The septal necks 

 (fig. 749, sp) are prolonged backwards to such an extent that each 

 becomes inserted into the mouth of the neck of the septum next 

 behind, thus forming a complete and wide tube which encloses the 

 siphuncle. This siphuncular tube is marginal or sub-marginal, and 

 may attain a diameter of one-half of that of the shell. The siphun- 

 cular tube is provided internally with a series of funnel-shaped 

 sheaths (fig. 749, sh), which are not very numerous, and occur at 

 irregular intervals. These were termed by Hall " embryo-sheaths," 

 upon the belief that they were connected with reproduction ; but 



sh 



~*P 



Fig. 749. — -A, Transverse section of the shell of Endoceras ; b, Vertical section of part of the 

 Shell of Endoceras Wahlenbergi, from the Ordovician rocks of Sweden (after Foord) : s, Cavity 

 of the siphuncle; sp, One of the "septal necks"; sh, One of the siphuncular sheaths; en, 

 Endosiphon. 



their true nature is not absolutely certain. In well-preserved ex- 

 amples, the great siphuncular space contains in its interior a cylin- 

 drical slender tube or "endosiphon " (fig. 749, b, en), which probably 

 represents the calcified wall of the proper siphuncle. With the 

 doubtful exception of one Silurian species (E. ? Ommaneyi) the genus 

 Endoceras is confined to the Ordovician rocks. Endoceras IVahlen- 

 bergi (the E. duplex of many writers) is abundant in the Ordovician 

 deposits of Scandinavia and Russia, and sometimes attains a length 

 of six feet or more. Casts of the great siphuncular tube are not 

 uncommon, and have the form of cylinders marked with oblique 

 annulations. 



In the genus Piloceras the shell is more or less broadly conical, 



