ESPECIALLY MYCTOPHOIDS 247 
of the caudal skeleton. This complex is remarkably similar in all four groups and 
undergoes identical fusions and reductions. 
Other trends exhibited by the salmoniform derivatives also separate them from 
the elopiforms, for example the constellation of characters associated with the paired 
fins and increased manoeuvrability. However fin spines are produced in the more 
specialized derivatives of both lineages (Marshall, 1962, has demonstrated the 
presence of true fin spines in the Heteromi) being due to the stresses imposed on the 
fins by increased momentum and bodily flexion (Patterson, 1964). 
Thus after the separation of the elopiform and salmoniform lineages at the level 
of the pholidophorid halecostomes the salmoniforms appear to have evolved possibly 
in fresh water for some considerable time. During this period many changes were 
effected within the Salmoniformes. Their subsequent radiation back into marine 
environments in the Lower Cretaceous involved much ‘ experimentation’, both 
successful and unsuccessful, in respect of survival to the present day. These 
evolutionary ‘ experiments’ have all been along somewhat similar lines with the 
same changes and deletions occurring time and again. These changes have been 
manifested in all parts of the skeleton, the skull, vertebral column, paired fins and 
girdles, and lastly in the caudal skeleton. However the majority of groups have 
also exhibited changes and specializations peculiar to themselves as well as showing 
the general trends present in all of the groups. Thus a variety of radiations occurred 
into particular ecological niches. Certain of these radiations are represented by the 
salmoniform suborders Cimolichthyoidei, Enchodontoidei and Halecoidei. In many 
groups the specialization for a particular mode of life has proceeded to such an extent 
that often any evolutionary plasticity exhibited by the parent stock is lost. Whilst 
filling their ecological niche successfully (success being based on their relative 
abundance in the fossil record) these forms seem to have been adversely affected by 
competition from later radiations. These later forms have advanced to a greater 
degree of efficiency in many aspects of life but still seem to have retained a more 
generalized overall construction and are still capable of further variability. This 
competitive aspect presumably accounted for much of the extinction of groups 
successful through the Upper Cretaceous. 
VI. ACKNOWLEDGEMENTS 
I wish to thank Dr. E. I. White, F.R.S., of the British Museum (Natural History) 
for the opportunity of studying the collections which form the basis of this work. I 
am most grateful to Dr. B. G. Gardiner of Queen Elizabeth College (London Univer- 
sity) for the supervision of my work and for much valuable advice ; to Professor G. 
Chapman in whose department the work was done ; to Dr. P. H. Greenwood and 
Dr. C. Patterson of the British Museum (Natural History) for much fruitful discus- 
sion ; and to Mr. H. A. Toombs and Mr. C. I. Macadie also of the British Museum for 
their generous help. Grateful acknowledgement is also made to the Central Research 
Fund of London University for a grant enabling me to visit many European Museums 
and University departments. Finally I extend my thanks to Mrs. A. E. Jordan of 
Queen Elizabeth College (London University) for valuable help in photographic and 
preparatory techniques. 
