6 



ACCELERATION OF DEVELOPMENT IN FOSSIL CEPHALOPODA 



But even in these, while there is in general a constant increase in 

 size of the successive mature forms, there is a much more rapid decrease 

 in size of the corresponding grovi^th stages. This fact is illustrated by 

 the accompanying diagram, showing a constantly lengthening ontogeny 



FIG. 2. 



-^ 



as more stages must be passed through before maturity is reached. 



The contrast between the size of mature Goniatites of the Paleozoic 

 and that of the goniatite stages of later Mesozoic Ammonites is even 

 greater; see for example the development of Goniatites of the Carbon- 

 iferous (PI. I, figs. 1-9), and of Placenticeras, (PI. XIII, figs. 22-28), a 

 Cretaceous genus. The same thing is seen in the development of the 

 genetic series leading up to Columbites of the Lower Triassic. Its imme- 

 diate ancestor, Gastrioceras, of the Carboniferous, when mature might 

 reach a diameter of several inches, as shown on PI. I, figs. 10-14; but 

 the adolescent Columhites, (PI. IV, figs. 1-10), ceased to resemble Gastrio- 

 ceras at a diameter of about ten millimetres. And Tropites, a still later 

 descendant of the same stock, in the Upper Triassic, ceased to show the 

 gastrioceran characters at a diameter of three millimetres, (PI. IV, 

 figs. 11-21). 



FIG. 3. 



