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. 1 shows a cross-section of an adolescent sheil, 
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 Schloenbachia 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, Glyphioceras, 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 Ammonoidea, 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. 
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