266 



MOLLUSCA. CEPHALOPODA 



SUB-ORDER II. AMMONOIDEA 



The Ammonoids are quite extinct and include the 

 ammonites, goniatites, etc. The shell is generally coiled 

 into a plane spiral, and as a rule the sutures show com- 

 plicated patterns (fig. 107). The siphuncle is placed at 

 the margin of the shell — usually at the outer, but occa- 

 sionally at the inner margin ; it is usually more slender 



Fig. 107. A, suture of an Ammonite {Parkinsonia dorsetensis) from the 

 Inferior Oolite. B, suture of Ceratites nodosus, from the Muschel- 

 kalk. I, one half of the external lobe; 1Z, 27, superior and in- 

 ferior lateral lobes ; al, auxiliary lobes; s, external saddle; Is, 2s, 

 superior and inferior lateral saddles ; as, auxiliary saddles. In each 

 case the straight line on the left represents the position of the 

 siphuncle at the external margin, and the curved one on the right 

 the line of contact with the next whorl. 



than in the Nautiloids and does not contain internal 

 calcareous deposits. The septal necks in the ammonites 

 are directed forwards, except in some of the earliest 

 chambers ; in . Glymenia, on the other hand, they point 

 backwards as in the Nautiloids ; and in the goniatites a 



