EVOLUTION OF FOSSIL CEPHALOPODA. 255 



divided by a secondary lobe (Plate III, Fig. 11), and the 

 whorl becomes higher and the spiral wider (Fig. 12). 

 Shortly after this the lobes and saddles all become 

 slightly digitate (Fig. 13), and the family relationship 

 of the young shell are unmistakable. 



Fig. I shows a cross-section of an adolescent shell, 

 four whorls, in which the broad, low helmet-shaped in- 

 ner whorls, the widening of the umbilicus, increase in 

 height of the later whorls, development of the keel, and 

 flattening of the sides are shown, seven and a half times 

 enlarged. 



Fig. 2 shows an adult cross-section, six whorls, one 

 and three quarter times enlarged, showing the angular 

 shoulders and considerable involution of the adult 

 shell. 



Since Schloetibachia appears near the time of final ex- 

 tinction of the ammonites, and is still normal in develop- 

 ment, it gives in its own development an admirable 

 epitome of the history of the race. And by combining 

 this with the ontogeny of its ancestor, Glyphwceras, we 

 are able to trace the genealogy with certainty back to 

 the first ammonoids that appeared in geologic history. 



By following this method the complete ontogeny of 

 any species of ammonite may be worked out, and in 

 order to learn the phylogeny of any form it is only 

 necessary to combine this with comparative study of 

 antecedent genera and species. When this is done for 

 all the Amtnonotdea, their genealogy will be more per- 

 fectly known than any other family tree possibly can 

 be. If evolution needs any demonstration to raise it 

 from a working hypothesis to a fixed principle of biology 

 we have it in the history of the fossil cephalopods. 



