8 



ACCELERATION OP DEVELOPMENT IN FOSSIL CEPHALOPODA 



Lost Stages. 



But recapitulation in later forms is by no means so simple as sug- 

 gested by the diagram given above ; the ontogeny is abbreviated, and the 

 successive forms do not repeat their full history. There is a constant loss 

 of stages or characters all along the race history, they being pushed back 

 and crowded out of the ontogeny, as Hyatt expressed it. 



All Goniatities must have sprung from a Bactrites radicle, but only 

 one, Agoniatites, shows a Bactrites stage. No later genera have even a 

 reminiscence of it, so completely is it lost from the ontogeny. Probably 

 all later Goniatites had for their ancestor the group of Anarcestes, and 

 yet the Anarcestes stage persists only in Devonian and a few Carbonifer- 

 ous genera, being lost, or buried, in the development of later groups. 



The first stage of growth in the shell of all Ammonoids is the pro- 

 toconch ; this is an adapted form, suitable to life in the egg, not corres- 

 ponding to any ancestral form, yet remaining the same in all genera. It 

 even keeps its minute size, about a half millimetre in diameter, whether 

 the mature form is a pygmy of half an inch or a giant of six feet. The 

 earliest stages of growth of several genera of Ammonites are shown 

 on PI. XIII. 



Much of the ancient history is gone through while the animal is in 

 the egg, and thus obscured or even obliterated, even in living forms. In 

 fossil forms it is wholly lost to us. And after the embryonic stage is 

 passed, it is advantageous to the young animal to shorten, or at least, not 

 to prolong, the larval development, during which it is helpless and at the 



