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determining to which of two converging species a given figure 
may belong. 
In answer to further enquiries Mr Buckman pointed out 
that Tetrabranchiate Cephalopoda were first a straight cone, 
then a curved cone, then a cone coiled upon itself with a whorl 
just in contact, and lastly coiled with the whorls very much 
overlapping. The changes which had taken place in the 
descent of one species of Ammonite from another were:— 
A progression from extreme evolution to extreme involution, 
and then in some cases a retrograde movement. 
An advance towards involution means generally an increase 
in the width of the lateral area; and this entails a decrease in 
the proportionate width of the ventral area and a fewer number 
of coils to reach a given diameter. 
Changes in the suture-line correspond with changes in the 
shape of the whorls ; but these changes take place subsequently 
to the whorl-changes, because more elaborate sutures are 
required to support the increased whorl-surface. An increase 
of the lateral area means an increase in the size of the lateral 
lobes and the production of more auxiliary lobes, while the 
ventral lobe is decreased by the law of compensation. Simi- 
larly—in some species, to compensate for a very elaborate suture- 
line, the spines and ribs disappear, giving place to a smooth 
test. Accessory lobes, it was pointed out, were not needed 
when the whorl was in the form of an arch from one lobe to 
another; but when, owing to the flattening of the side con- 
sequent upon increased involution, this part of the whorl became 
as it were suspended, then an accessory lobe was put forth to 
support it in the middle. 
Finally, that changes among Ammonites were rendered 
possible by the following rule:—The assumption (by the 
descendants) at an ever earlier age of the characters of their 
adult ancestors. 
A study of the evolution of Ammonites brings out two 
facts clearly :—That, generally speaking, those which have gone 
through most changes die out the soonest; while those which 
have gone through the fewest changes are the parents of the 
future generations, 
