12C INFERIOR OOLITE AMMONITES. 



I retained the scope of their divisions, I was practically according them a totally 

 different value. For several reasons the arrangement then put forward is objec- 

 tionable ; not the least important of which is that several of the divisions as 

 constructed by older authorities and retained by myself (but with different rank 

 on account of the greater subdivision) are not at all natural. I wish to erase this 

 plan in toto ; and I consider that it will be more in accordance with scientific 

 requirements to follow the method put forward by Zittel, 1 and to group the genera 

 into families (these families would be of about equal scope to what I called 

 " generic groups "). At the same time I do not propose to follow Zittel's nomen- 

 clature in all places, but only the method of arrangement. As to what should 

 constitute a family it is not easy to determine ; and this will probably be a 

 debateable matter for some time to come. The idea which is proposed for this 

 work is to group in a family those genera which have, besides similarity in 

 character, 2 a common origin, and which therefore realise to the full the meaning 

 of the word family. 



It may be advantageous to indicate Zittel's mode of division 3 — as follows : 



CEPHALOPODA. 



Order— Tetrabranchiata . 

 Sub-order — Ammonoidea . 



Divided into — 



A. Retrosiphonata. 



B. Prosiphonata. 



B. divided into — 



Group I. Latisellati. 

 Group II. Angttstisellati. 



Group II divided into ten families, of which the eighth is Harpoceratidce. 



1 ' Handbuch der Palseontologie,' Bd. ii, Abth. i, 1881-85. 



2 It must be obvious tbat those genera which have a common ancestor, such as an Arietites, must 

 possess a certain similarity of character, and should differ from those genera whose ancestor is an 

 JEgoceras or an Aniallheus ; but it happens that the matter is complicated in two ways, — (1) by 

 convergence in shape of members of two families advancing towards each other, so to speak, from 

 opposite directions, — (2) by certain species of one family being apparently mimetic of those of another. 

 "We must, therefore, in arranging our families, avoid those errors which a consideration of species 

 falling under the above two headings might lead us to commit. 



3 Op. cit., pp. 332, 411. 



