INTKODUCTION. 
XXXIX 
It seems inevitable that either some Mesozoic genera must he 
accepted as Trepostomata or that Order must be redefined. 
The Order Cyclostomata becomes more homogeneous when the 
Trepostomata are excluded, but it is still so large that it is 
advisable to arrange the families into suborders. 
It is possible to frame several working classifications of the 
Cyclostomata, each based on different assumptions as to the character 
of primary importance. Thus the nature of the zoarium, the 
general shape of the zooecium, the linear, radial, or irregular 
arrangement of the zooecia, and the solid or cancellous structure 
of the skeleton, might each be used as the primary systematic 
character. The final test between such classifications is the 
liistorical. "When the many wide gaps in the succession of the 
Mesozoic Bryozoa are filled, we shall know which were the ancestral 
forms, and shall be able to arrange the genera in the order of 
their descent. 
Any classification must at present be experimental, and the test 
by which it must ultimately be judged is its agreement with the 
actual facts of succession and descent shown by the geological 
distribution of the genera. The historical test is the final test in 
phylogeny for organisms with sufficient skeletons to give abundant 
fossils. 
Xo doubt the palaeontological record of the Bryozoa is so imperfect 
that it will be long before this method can be fully used. But 
every effort to adopt it calls attention to the gaps in the evidence, 
and thus tends to remove them. In the meantime we must be 
prepared for tentative suggestions, and progress will be achieved 
by those who are ready to propose reforms, even though they 
thereby risk mistakes. 
In proposing changes in the classification of Cyclostomata in 
1896 and 1899, I was quite prepared to modify the schemes 
suggested with further knowledge ; but some definite classification 
seemed necessary in order to demonstrate the relation of the 
successive Cyclostomatous faunas of the Mesozoic and Cainozoic eras. 
The classification may be based on zoarial or zooecial characters, or 
on both. The last of these three courses seems to me the best, the 
zooecial character being generally used for the suborders and the 
zoarial for the families and genera. 
