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Approximate phylogeny of the families of Cephalaspidea. 
The group of families on the left side are the most primitive of 
recent Tectibranchiata; the median and right hand groups being 
far more specialized, and more remote from the Notaspidea and 
Anaspidea. 
A phylogenetic table of the shell-bearing Opisthobranchs has been 
given by M. Cossmann in a work of great merit, ‘‘ Essais de Paléo- 
conchyliologie Comparée” (1895), derived mainly from his studies of 
the fossil forms. The great discrepancy between the results obtained 
by the distinguished French author and myself, are in part trace- 
able to the widely different material studied, and in part to the fact 
that Tectibranchs, like Pulmonates, are singularly non-committal in 
the characters of the shel/. In fact, I feel that it is not extreme to 
state that the shells alone, in either group, are totally inadequate to 
express the affinities of families and genera. In so many groups are 
the shells more or less degenerate, so many are the cases of parallel 
or converging development of the shells, that conclusions based upon 
them alone, without a knowledge of the soft anatomy for a primary 
guide, are practically valueless for the appreciation of the affinities 
of genera and families, either in Tectibranchiata or Pulmonata. 
There can be no doubt, however, that paleontology will prove of 
great value in supplementing the evidence of comparative anatomy ; 
and the best results can only be obtained by a union of the two 
methods. 

