AMMONOIDEA. 



849 



Fig. -&i.—Stephanoceras Brackenridgii, viewed laterally 

 and in front, showing the extensions of the margin of the 

 aperture. Jurassic. 



The aperture of the shell in the Ammonoids is commonly fur- 

 nished with lateral extensions or " ears," while its ventral or external 

 side may be prolonged into a long pointed process or lobe. In this 

 way a kind of cowl is 

 sometimes formed above 

 the aperture, and the 

 width of the opening 

 may be considerably 

 contracted by the in- 

 ward bending of these 

 lobes (fig. 763). In the 

 Goniatitida and Clyme- 

 niidce the aperture re- 

 sembles that of Nautilus 

 in being simple, with a 

 ventral sinus. In many 

 cases the shell exhibits 

 at intervals transverse 



ridges or contractions ("varices"), which represent the periodically 

 formed mouths of the shell. 



The ve?itral and dorsal sides of the shell of the Ammonoids 

 cannot be always determined with certainty. There are, however, 

 strong grounds for considering 

 the external or convex side of 

 the bent shells to be ventral, 

 while the concave or internal side 

 is dorsal. 



The siphuncle in the Ammon- 

 oids is always cylindrical, and is 

 never furnished internally with 

 secondary calcareous structures re- 

 sembling the " obstruction-rings " 

 of many Nautiloids. Usually the 

 sheath of the siphuncle is calci- 

 fied, so that a continuous shelly 

 tube is produced. The siphuncle 

 commences by an inflated por- 

 tion (fig. 764, a\ Which indents Fi § ;■ ^--Section of A nmonites {Cosmo- 

 , / » . ceras) Farkmsom, in the mesial plane, show- 

 the prOtOCOnch (pr) On One Side ; ing the "prosiphon" (/), the siphuncle (si) 



d. . i . , . -i with its dilated commencement (a), and the 



It IS, tO begin With, either protoconch (J>r). (After Munier-Chalmas.) 



internal or central in position. 



In the majority of the Ammonoids the siphuncle ultimately comes 

 to be placed on the external margin of the shell (fig. 764, si), but 

 in the Clymeniidce it remains internal. The septal " necks " of the 

 Ammonoids are variable in their development, as also in position. 

 vol. 1. 3 h 



