MOLLUSCA : CEPHALOPODA. 



433 



or "septa," the edges of which, where they appear on the 

 surface of the shell, are termed the "sutures." 



3. The outermost chamber of the shell is the largest, and is 

 the one inhabited by the animal. 



4. The various chambers of the shell are traversed by a 

 tube, termed the "siphuncle." 



Agreeing in all these fundamental points of structure, two 

 very distinct types of shell may be distinguished as character- 

 istic of the two families Nautilidce and AmmonitidcE^ into which 

 the order Tetrabranchiata is divided. 



In the family Nautilidcz (fig. 234, dfand e), the "septa" of 



Fig. 234. Diagram to illustrate the position of the siphuncle and the form of the septa 

 in various Tetrabranchiate Cephalopoda. The upper row of figures represents 

 transverse sections of the shells, the lower row represents the edges of the septa. 

 a a A mmonite or Baculite ', b b Ceratite ; c c Goniatite ; d d Clymenia ; e e Nau- 

 tiltts or Orthoceras. 



the shell are simple, curved, or slightly lobed ; the " sutures " 

 are more or less completely plain ; and the " siphuncle " is 

 central, sub-central, or internal (t.e., on the concave side of the 

 curved shells). 



In the family Ammonitida (fig. 234, a, b, and c\ on the 

 other hand, the septa are folded and complex; the sutures are 

 angulated, zigzag, lobed, or foliaceous ; and the siphuncle is 

 external (/.&, on the convex side of the curved shells).* 



In both these great types of shell, a series of representative 

 forms exists, resembling each other in the manner in which the 



* In the Ammonitida, the initial chamber ("ovisac") of the shell is an 

 egg-shaped chamber isolated from the first air-chamber by a distinct con- 

 striction, whereas no such arrangement obtains in the Nautilidce. Such a 

 structure, however, is found in Spirula, Belemnites, and other allied forms ; 

 and it has recently been concluded (Munier-Chalmas) that the Amman- 

 itidce are properly Dibranchiate, their shell being an internal skeleton or 

 phragmacone, similar to the shell of the Spirula. 



2 E 



