INFERIOR OOLITE AMMONITES: 

 SUPPLEMENT: 



I. REVISION OF, AND ADDITION TO, THE HILDOCERATID^E. 



A thorough revision of the genera and species belonging to the family 

 Hildoceratidse is imperative, for two reasons : — (1) because of the large accumu- 

 lation of material, with so many new forms; (2) because the knowledge concern- 

 ing the affinities of different species has necessarily become more complete. This 

 revision will form the main object of the present Supplement. 



The great difficulty in connection with the Hildoceratidas is the occurrence of 

 many series of specimens with a remarkable similarity of shape. So long as such 

 a feature was regarded as of supreme importance, and due consideration was not 

 given to other characters, because their significance was misunderstood, so long 

 was the classification of the Hildoceratidse incorrect and unsatisfactory. 



Outward form is of merely subordinate value in classification. It is obvious 

 that species most dissimilar in mere shape can produce, b}^ modification, specimens 

 which are very similar. Hence some other criteria of affinity must be sought for; 

 and they are to be found in the characters of the ornamentation and suture-lines 

 of the species. In comparison with the mere shape both these features are 

 constant. They are modifiable and they are modified ; but they change so much 

 more gradually than does the mere shape of the conch that they may, for practical 

 purposes, be regarded as fairly stable. At any rate, they change so slowly tbat 

 their identity is preserved. 



In regard to the suture-line and the character of the ornamentation as criteria 

 of affinity, it must be acknowledged that the former undoubtedly holds premier 

 position. But there are many practical reasons why it cannot always be used as 

 efficiently as it deserves. And when, as in the Hildoceratidae, the ornamentation 



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