fXTRODlTCTIOX. 
Thk Cretaceous Bryozoan Fauna. 
The Cretaceous Era is of special iiuportance to students of Bryozoa^ 
as it was practically the birth-time of the chief modern types of 
this class of animals. It is the era in the history of Bryozoa that 
corresponds to the Carboniferous with Echinoidea and to the Eocene 
with Mammalia. It the former method of separating the older 
from the modem types into such divisions as Palaeocrinoidea and 
Neocrinoidea, or Balechinoidea and Euechinoidea, were adopted for 
Bryozoa, the separation between Baloeobryozoa and ^N'eobryozoa 
might be drawn between the Jurassic and the Cretaceous, and not 
in the great stratigraphical gap between the Permian and the 
Trias; for the existing groups of Bryozoa first became abundant 
after the great extension of European seas during the Lower 
Cretaceous. 
The most characteristic Cretaceous Bryozoa belong to the Cyclo- 
stomata, and the members of that Order in the present seas are 
a comparatively few isolated survivals from the rich Cretaceous 
fauna. The gaps between the various living Cyclostomata are so 
wide that it is impossible from them to trace the phytogeny of this 
Order or to frame an adequate classification of it. ISTeither the 
embryology nor the morphology of the recent species can give the 
same help with the Cyclostomata as with the Cheilostomata, 
for in comparison with the living Cyclostomata, the Cretaceous 
are so abundant in species and so varied in structure that 
the usual definitions of the Order based only on the recent 
fauna are useless. Thus Busk’s definition of the Cyclostomata, 
as revised by him in 1875, is as follows^: -‘‘Cells tubular, 
calcareous, partially free or wholly connate ; aperture terminal, not 
furnished with a movable lip or fringe.” There is nothing in this 
definition to separate the Cyclostomata from the Trepostomata, and 
the terminal position of the aperture, the main distinction from 
‘ ( t . Eu>k. Cat. Mar. Polyz. Bnt. ]\rus. pt. iii., Cycl. 187o, p. 2. 
