GENERAL ZOOLOGY 



Fig. 20.4. Fossil remains of two genera 

 of ammonoids, which Hved in periods 

 separated by almost 200 million years. 

 .4, Gomatites, of the Lower Carboniferous 

 (Mississippian); note the relative sim- 

 plicity of the contours of the septa. B, 

 Placenliceras. of the Upper Cretaceous; 

 the septa in this genus are extremely 

 complex. In both of these specimens 

 the outer layers of the shell are almost 

 completely lacking; a little of the shell 

 remains near the broken outer end of 

 the Placenliceras fossil. (Photographs 

 courtesy Ward's Natural Science Estab- 

 lishment, Inc.) 



Ammonoids are known only from their fossilized shells, or exoskeletons; these 

 demonstrate a long succession of forms culminating in large and highly elabo- 

 rate shells. Ammonoids occurred in such numbers and in such a wealth of 

 species that during the Jurassic and Cretaceous they were probably the 

 dominant forms of marine invertebrate life. Later, at the close of the Cre- 

 taceous, this entire subclass of cephalopod mollusks became extinct; there are 

 none in the modern fauna, and no fossils of ammonoids are known subse- 



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