266 AN INTRODUCTION TO THE STUDY OF FOSSILS 



3. On shell i, (c) follow a septum from its center to its outer 

 edges, — the sutures ; note it gradually changing from the 

 simple, fiat septum into the very complex lobes and saddles of the 



Fig. 118. — The ammonite, Placenticeras intercaiare Meek, from the Pierre forma- 

 tion (Cretaceous) of South Dakota. A, side view of specimen (x 5). B, aper- 

 tural view. C, suture of same from siphuncle (upon the outer edge of the coil) 

 to umbilicus (umb.). si., siphuncle. (From Meek.) 



sutures. ( A section of any ammonoid shell will show the same 

 change.) Compare this with a similar section through a fossil 

 Nautilus ; what difference do you note ? 



4. What principally distinguishes the Ammonoidea from the 

 Nautiloidea ? 



