

THE ORIGIN OF MAN 71 



As I have said, the Mesozoic seas were charac- 

 terized by an abundance of the shelled cephalo- 

 pods known as ammonids. These beautiful coiled 

 shells, relatives of the nautilids, were exceedingly 

 varied in size, shape, ornamentation and in the 

 character of their septa. They exceeded the 

 Paleozoic nautilids as much in size as they did in 

 specific differentiation. They also appear to have 

 been more active animals and better swimmers 

 than the nautilids and therefore crawled less over 

 the bottom of the sea. The belief that they did 

 swim well is deduced from the nature of the very 

 thin shells, the wide distribution of some of the 

 species, and the small depth, or narrowness, of the 

 coiled cones. The shells are somewhat like those 

 of the nautilids but they differ in a number of 

 respects. They are nearly always more ornate, 

 narrower or less deep, and are often distinctly 

 keeled along the center of the outer whorls. Also 

 the mouth of the shells often has lobed extensions 

 on the sides and there is also a median keel that 



